Just thought I'd update; I removed the driftwood from the tank and the frog was alive and unharmed. Sagat still sometimes stalks and even nips at him but he doesn't seem to be any the worse for it, physically at least. In fact, if he's still, Sagat might go investigate him but won't flare or chase; I've noticed that when feeding him, too; he only takes interest in things that are moving. Feeding was a different story; Sagat not only would nab the food long before I could get it anywhere near the frog, but would become very aggressive with the frog when it finally got a piece. Still not harm to the frog, but obviously that was going to be a problem. I used a net to hold Sagat off to the side of the glass while I fed the frog with tongs. He took to it slowly but seems to enjoy it and it's by far the most active and comfortable he's seemed the whole time. After that, he spent most of his time hiding out in the plants. Netted him up and left him back yesterday and put the wood back in. I probably could have got another 2 frogs and made it work (maybe try some pellets that the fish won't be interested in or get a breeding box and feed him in there) but it's a lot of work-arounds and in the meantime, I suspect Sagat would still be aggressive towards the frogs if they moved about much.
Well, that was my first ADF experience; ended much better than my first snail experience. There's probably something to be said about keeping territorial fish on their own, adding nothing but food for months and then trying to introduce a tankmate.