would something painted with spray paint be safe for my tank?

I would have to say no to putting anything with spray paint on it in your tank. Spray Paint contains lot's of harmful chemicals that won't be any good for your fish.
I'm not sure of any place that I can link you to for aquarium safe paints at the moment, maybe someone else can shed some light on the subject.

I'm guessing you are talking about pvc cement? yes when that is fully cured it should be fine. What you can do also instead of painting your pvc is either use black pvc or use "GE Silicon I" which is safe for the fish and smear it on then put your gravel on it. Let it cure for 24 hours and it should be ready for the tank. If you take a look at my pictures you'll see a pvc that i put a coat of sand on with silicon.

Are you talking about painting the riser tubes? These should be the only part of an UGF visible. The substraight glued to whatever with aquarium silicone sounds like a great idea and those fish houses look pretty cool Serial32.