The last few days (since Thursday), I've been soaking Gus's foot with Epsom salts and then applying a Magic Cushion wrap. Besides the fact that the MC is so sticky (it only comes off with alcohol, thanks CoTHers for that tip), it works really, really well.
Here's some photos. The black hole just to the right of the tip of the frog is where the abscess was dug out from. This photo is post-soaking.
I forgot to take some photos of the wrap before I turned Gus back out. So here's my handiwork. It's definitely not the prettiest, but it's practical. And... it lasts 24 hours outside to boot. Whohoo!