This is a great idea! We have one next to our BP's tank now and it does wonders for the humidity. Also, having the snake's enclosure in a smaller room helps, too. It will be easier for temps and humidity if you can control the room somewhat, too.
I think we gave our BP a URI by misting too much to make up for humidity drops. She's almost done her course of antibiotics now. But we won't make the same mistake again.
Yes, as Pat says, wet does not equal humidity. Like I said above.
If the belly doesn't get better within a day or so after drying him out, take him to a vet - or sooner if he seems to react when you touch/move him (like he's uncomfortable or in pain).
Good luck!