When I first read your idea about the floral foam, it sounded like a great idea. I do think it would help with humidity a lot. I think part of what it is supposed to do is to suck water up and distribute it throughout the foam block, which would then have tons of surface area for evaporating and creating humidity.

But the more I think about it, the more concerned I am getting about it being a perfect breeding ground for bacteria. I used to be really into keeping aquarium fish, and most filtration systems make use of some sort of foam, because it has tons of surface area for establishing the beneficial bacterial colonies that you need in a fish tank.

I know you said you'd just toss it and replace it with a new one each time you disinfect the tank. If you are talking about doing that once a month, that is going to be plenty of time for gobs of bacteria to have grown in the floral foam. If you decide to try it, I'd recommend that you do something towards cleaning the foam every few days, to at least slow down the growth of bacteria. I'd be slightly concerned with using bleach since I suspect that foam will be hard to rinse thoroughly. Vinegar might be a good bet since it has less potential to be harmful but still does a pretty good job as a disinfectant. Also I have no idea about the chlorhexidine (or something similar) that some people use to disinfect. I think I read that you don't need to rinse after using it?