How does a Dispel Magic attached to a Hallow or Unhallow effect work?

Question

Hallow and Unhallow (in D&D 3.5e and Pathfinder) allow a second spell (from a specific list) to be fixed to the site, and has a duration of 1 year.

Dispel Magic normally has a duration of instantaneous, and therefore its rules are described based on the assumption that it only functions when it's first cast.

So how does Dispel Magic work when it's given a duration? Is it cast once on valid targets that enter the area, or is it continuously recast each round? If the check fails to dispel, is the specific target creature or effect immune for a duration (such as 24 hours)?

I would understand if Hallow and Unhallow stated that they cast the specified spell on any creature entering the area, but they don't; they say only that the spell acts as if it had a duration of 1 year.

Answer 1

The Dragon #338 Sage Advice column "Official Answers to Your Questions" includes these exchanges:

Question 1

When I affix a dispel magic to my hallow spell, what type of dispel effect is it—a targeted dispel, area dispel, or counterspell? Can I choose when I create the hallow?

Answer

A dispel magic fixed to a hallow or unhallow spell can be treated only as an area dispel, not as a targeted dispel or a counterspell.

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Question 2

When a spell effect is fixed to a hallow spell, how do unusual durations (such as those of protection from energy) or instantaneous durations (such as those of dispel magic) work?

Answer

The spell effect fixed to a hallow or unhallow spell is treated as being cast on any eligible creature each time it enters.

In the case of “ablative” spell effects that are used up gradually (such as aid or protection from energy), the full effect of the spell is renewed each time the eligible creature re-enters the hallowed/unhallowed area (with the newer version entirely replacing the older version). In the case of instantaneous effects (such as dispel magic), the spell affects eligible creatures each time they enter. (84-5)

(Links mine.) The Sage at the time is Andy Collins. These exchanges are repeated in the D&D Frequently Asked Questions (82-3), with which some folks have issues. While in general such rulings can be controversial, these rulings make about as much sense as anything else, and the ruling have the added advantage of actually coming from one of the revised game's designers rather than, for instance, the dude who was answering the phones that day (a risk one takes with some of Wizards of the Coast's customer service rulings).

A quick search of Paizo's site revealed no immediate and stunning insight into or changes to the operation of the spell hallow for Pathfinder, so unless contradicted by a later designer (in which case I'll happily update this answer), these rulings seem equally functional in that game.