I keep my seeds in those cheap foam coolers you find in the summer. If I want to keep a room dry, some air conditioners are good at doing that. Mine has a dry setting, which dries out the air (I think it actually cools better than the regular setting, too, when it's humid in the house). You could put a pouch of diatomaceous earth in the cooler (and/or in your seed bags) to absorb water, if you wanted. I haven't tried that in the cooler, yet. If you put it in the seed bags, expect it to get on the seeds, and make them white, however (I have done that, but I mostly did it to protect them against insects that were hatching on some squash seeds).

So, the coolers won't keep the seeds cool, exactly, but they should protect them against quick fluctuations of temperature, and block out the light.

Fans can also help to keep a room dry. While they don't necessarily reduce the humidity, it does seem to protect items from gathering moisture.

I would think burying the seeds could be effective, as long as they're in a protective container that won't get wet or attract pests, and as long as they're deep enough. If you have clay-type soil, it'll probably insulate better than sandy soil, though.

I know seeds have been known to stay viable for a really long time in clay pots in caves. Burying them might be similar to that, depending.