IndyPlanet got back to me! They say I would basically have to accept the profits like normal, and then just donate it all myself. So it's an extra step, but I guess it wouldn't be much hassle if I keep track of weekly or monthly sales and donate the exact amount like clockwork.
That's assuming the anthology sells any copies.
And that's assuming the anthology gets off the ground.
So far there's four of us on board, and I don't know how long my comic would be. I generally draw my comics until they're done. So even with only a handful of people we could still do it, if everyone did a lengthier piece or contributed more than one comic.
I'll let this stew a while longer and see if more people bite.