How much of a person or object must cross an extra-dimensional space's interface from inside, in order to attack a target outside of said space?

Question

The party is in the habit of using extra-dimensional spaces such as Rope Trick or Psychoportive Shelter. Recently they want to use it as a sniping blind or bunker.

As per the description of either, people inside the space can see out, but not the opposite. Powers and spells cannot cross the interface, neither can area of effects. They are wanting to stick their hand out and manifest/cast stuff at enemies, as well as shoot or throw things at enemies from inside the space, and then withdraw their hand/weapon so they can't be attacked. (The 5' movement rule is being proposed for that part.)

Both the Rope Trick and the Psychoportative Shelter entrance can become invisible (handy for sniping). The former requires the rope to enter, but the latter doesn't seem to limit who can enter, assuming that you can see the interface once it is invisible.

The Rope Trick interface is a 3'x5' window, limiting how many could use it effectively. However the Psychoportative Shelter interface is a 7' diameter plane, which gives considerably more room, not to mention that it is most likely to be vertically emplaced, unlike the horizontally emplaced Rope Trick.

Therefore, how much of a person or object must cross an extra-dimensional space's interface from inside, in order to successfully attack a target outside of said space?

Please note: this question is not addressing the meta-balance issues of this tactic - it may be quite effective at low levels, but once Dispel Psionics / Dispel Magic comes on the table, it's no big deal. There are also the Transdimensional Power & Spell feats available. Plus enemies can pull the same stunts. So balance isn't the concern. This question is intended more to address the action economy, mechanical rules, and order of events.

Answer 1

The mechanical-technical answer

The Rules Compendium on Out of Sight? No Problem! says

One of the most important parts of understanding the line of effect rules, and therefore making correct decisions during combat encounters, is how to determine the point of origin of an effect. For ranged attacks and other effects that originate from a creature, the point of origin is any corner of any square that the creature occupies. (80)

(Emphasis mine.) Thus a creature, according to these rules, just can't bypass this restriction by, from its space, poking its bits into a square the creature doesn't occupy so as to trace line of effect from those bits instead of a corner of one of its squares.

In other words, a creature that doesn't want to deal with vagaries wrought by the extradimensional space must itself exit the extradimensional space—either as a whole or in part so as to put one or more of its squares outside the extradimensional space—if it wants to establish line of effect beyond the extradimensional space for launching attacks or spellcasting.

(The sort-of-if-you-squint-hard exception to this is the ring gates (DMG 265) (40,000 gp; 2 lbs. for a 2-piece set) and similar items that specifically allow attacks and spells to be launched from afar, and those typically impose severe restrictions.)

Tried and tested house rules for consideration

This DM's house rule says that window as the spell rope trick uses the term (and sometimes when window is used elsewhere), is, in fact, a convenient metaphor for transparent surface. (That is, this DM believes a window that can be opened and closed and used for entering and exiting is, more properly, a transparent door.) With that in mind, this DM, for example, doesn't allow creatures camping inside the effect of a rope trick spell to open the metaphor to let out a cooking fire's smoke, nor, for example, does this DM allow creatures to fill the space with acid (or whatever) then crack the window to weaponize the ensuing rush. This ruling has saved this DM a lot of hassle.

Likewise, this DM has a house rule saying that creatures—unless an effect precisely specifies otherwise—can't be on two different planes simultaneously, he having applied the rules in the Forgotten Realms Campaign Setting on portals (59–61) to extradimensional apertures generally. That is, those rules forbid unattended objects from crossing portal boundaries and mandate that a creature pass entirely through an aperture to be on the aperture's other side, and, until the creature passes completely through, the creature's only on its starting plane, incapable of anything involving its destination plane. This house rule has saved this DM's sanity on more than one occasion. For example, this house rule prevents the aperture created by the 9th-level Sor/Wiz spell gate [conj] (PH 234) from being used to flood the dungeon or shoot lasers at foes (albeit the rules may forbid such a result anyway). Likewise, for example, this house rule meant that a PC that was in a bag of holding carried by a buddy could not keep his head out of the bag while the PC's buddy traversed a plane that possessed the trait fast flowing time—in short, this house rule meant the DM didn't have to rule what happened when the PC aged just his head.

Answer 2

I think the best approach here would be to use similar tactics allready discribed in the rules. An incorporeal creature emerging from a solid object seems like an OK match to me. The folloving is how I understand mentioned process and how it applies to your situation:

• Staying close to interior is probably similar enough to standing (or more probably sitting or lying) near the rope trick's extradimentional space entrance.

• You probably doesn't have to spend any kind of action to briefly stick some not very big portion of your body out of the extradimentional space to be able to affect Material plane, as well as to pull it back once you've affected it. This is based on the fact that there is no mentoin of incorporeal creature being ought to spend some actions to emegre from the surfase or to hide inside it again. It's just a minor shift in creature's position. As with an incorporeal creature, once you "emerge", you have cover.

• Someone can affect you during your action which affects Material plane (or in most cases right prior to this action). It requires immidiate or readied action, as it is now your turn.

This is mostly true for spells, as they can't cross the entrance. For ranged weapon attacks it is probably possible to just shoot in and out without emerging in any way. Those shooting in just can't see where are they shooting.

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As a side note I should point out that it may be not such an easy task to dispell extradimentional space when the rope is pulled inside. From the point of view of someone on the Material plane, extradimentional space has no "borders", and what the spell affects and probably persists on (the rope) is out of reach for dispelling magic. And it is questionabe at best if you can affect an entrance of extradimentional space with it.