Alright, the pet shop got me using birdseed as a substrate and the notions involved with it and arguemnts against other things seemed plausible, but after observing the "kids" for nearly a month with it, I'm seeing a point of weakness and concern that I haven't seen brought up-
Of course the cage is the reptiles litter box and clearly they're not terribly disciplined in ther behavior. I've seen some fairly damp fecal matter come out of these things and more interestingly 'recently' damp fecal matter. Unless it's on a stone and gets baked away, the moisture (etc.) leeches into the birdseed which is then... fresh for consumption (not by me.)
After the brief discussion on the "longevity" of sand, this seems like a particularly more insidious problem even with a frequent purge. This seems like an ideal place for bacteria to manifest as I'd expect Uro dung isn't sterile...
Sand is going in soon, anyhow, and it seems the substrate needs to be culled sometime during deworming as it is, so my long-term interest isn't specifically for my own situation. I figure birdseed will end up in a dish with iguana food to also try to get them consuming that.