The spells you're looking for are "Avoid Planar Effects" (Planar Handbook, cleric/druid 2, sor/wis 3) or Water Breathing
Which explicitly allows for a "temporary respite from the natural effects of a specific plane. These effects include ... lack of air." And it's reasonable to expect that a magical demi-plane counts as a plane.
The downside of this spell, is that while it can target one creature per level (which is almost enough for one casting of this to handle 8-10 scarabs at your level), it only lasts for one minute per level.
At cleric/2, it's cheap enough to get this invested in a custom magical device (or two) which can be emplaced into the bottom of the portable hole (and would make a quite decent long term investment, to be honest.)
Alternatively, a bottle of air per set of scarabs (depending on specific breathing rules and size adjudications) could work with a sufficiently complex bellows/clockwork mechanism. They are a touch pricey alone though.
You could also do something with filling a portable hole with enough water and casting water breathing on the scarabs, which, if organised correctly, should work quite well.
A specific working through for a level 6 group:
Let's work through this for your specific group. We'll assume the budget of 13,000 gp. (An important party goal, that everyone's prepared to tithe 1/5th of their wealth by level to).
It should last for a week.
We'll presume that the scarabs can count as "willing" for spells through some combination of wild empathy, charm monster, bribing with food, handle animal, etc.
A single wand of water breathing will provide 50*6 creature hours of water breathing at 4500 gp. Or, 30 hours for your entire group of "pets". Much worse for avoid planar effects.
Thinking about "the dream of metal" lead me to the thought of quintessence, which can take objects and living creatures outside the time stream. (Some amount can.... ::handwave::) We'll equate "extremely small" with the size category "fine", just as a pessimistic choice. Therefore, using the standard doubling rules, 2 oz for diminutive, 4 oz for Tiny, and 8 oz for small. That's 8 castings (total though, no recasting necessary) per creature. Hiring 4th level spells is 7*40 gp each, or 8*7*40 = 2240 gp to preserve a scarab. (You can get significant savings with metamagic, but I'm doing the most pessimistic view here.). Therefore, for 10 creatures, 22400 is out of budget, but close enough to be viewable from a distance. Also, for that many castings, it's probably "questing time" for the Shaper who's making all this stuff, so I would certainly consider it a viable option.
Or, if the party has enough time and ranks in handle animal:
Handle animal has an important clause: "You can use this skill on a creature with an Intelligence score of 1 or 2 that is not an animal, but the DC of any such check increases by 5. Such creatures have the same limit on tricks known as animals do." Therefore, so long as the group has someone with ranks in handle animal, they can spend a month or two teaching the group: "Seek (DC 20), Attack (DC 25), and Heel (DC 20) At level 6, with max skill ranks in handle animal, these dcs are quite reasonable.
Looking at Arsenic and Old Lace we're interested in a cost-effective multi-day poison.
First, if you have someone (either psionic or otherwise) with Minor Creation and the craft:posionmaking skill, you're completely set, for free. Otherwise, there are some useful cost efficiencies. Here you want to grow a field of striped toadstools:
"Craft DC 15/None: Striped Toadstool (DMG). Ingested: DC 11, deals 1 Wis/2d6 Wis and 1d4 Int. Presumably no craft check necessary for just the raw toadstool."
Therefore, with psionic minor creation, or access to the right biome, you can get a whole bunch of striped toadstools to keep your "patients" sedated for the duration of the trip.