vtol_guy - I think you'll be fine. People are just really paranoid right now. I do think it would be wise to first try this when no one is around, or at a private field just so you don't scare the sheepish people, or break any of the AMA laws of your regular field.

Now let's talk about why your plan might not work like it should. I really doubt that hobby rocket engines are made to strict enough standards that mounting them on the wing tips wouldn't cause an asymmetric thrust problem. However, I could be wrong since I've never tested any rocket engines on a test stand (well I once did when I was a young lad, but we won't talk about that ) So what I would do instead is get a small piece aluminum that's maybe 1/8" thick. This would be used to protect the belly of the plane. From there you could make a simple holster for the engines (I assume you want this to be "Cool" so two engines are a must )

This is just an idea that solves the heat issue, and asymmetric thrust issue. In any case, I would try to mount the engines as close to the fuselage as possible in case an engine fail to ignite.

Have fun, and be safe with your experiment. Don't let anyone tell you can't achieve something. Finally, the AMA laws are just for insurance purposes. You can break them all you want, but you won't be covered by their insurance. I get the feeling that a lot of people think the AMA is a government agency, and that you'll go to jail if your break their laws.