Krystal and I had an extensive conversation about this on the old caging forum and via e-mail. Unforunately, most of the relevant information was probably discussed on e-mail and I no longer have that info to share.
There is nothing wrong with this idea but you'll have some unique challenges.
First, the vinyl coating on the shelves will not accept very many types of sealers, if any.
What I would do is line the shelves with thin, expanded foam PVC - the same material used by Boaphile plastic cages.
This material is smooth, easy to clean, conducts heat well, and has a flame retardant. The latter feature is key if you have some heat tape melt.
You could leave a groove behind the cage or route a hole out underneath to put in an UTH or piece of Flex Watt.
You should also shim the PVC slightly to allow the UTH to slide underneath if you decided to cut a groove in back.
You'll need to add a substrate dam which could also be made out of the PVC.
However, I'd prefer to put in a wooden substrate dam and then build a sliding door off of it. This will be cheaper and easier to construct than a hinged door.
But nothing wrong with a hinged door if you prefer that.
The PVC is available and sign and plastic shops and will cost from $20 to $50 depending on what color and thickness you go with. The full name is expanded foam PVC (rigid).
We can discuss working with the PVC in separte posts.
Oh yeah, if the shelves can hold hundreds of lbs. of books they can handle your reptiles provided you seal them properly (the shevles, not the snakes 
I'm moving into a new house, and my room has some built in shelves that are somewhat similar to deep bookshelves. I was having another probably crazy idea about turning them into a built-in cage. The shelves are made out of what i think is fairly good quality pressboard. It is fairly thick, and has a very smooth finish on it. If i gave the inside of the shelves a thick coating of sealer, and hinged an acrylic door on the front, do you think the shelves would hold up? Good idea or bad? Sorry for buggin you guys with my ideas, and thanks for taking your time out to help!