I salvaged wings off a foamie and would like to cover them using Monokote but I'm afraid the heat will melt it. Has anyone successfully found a way to use Monokote on foamies ? Any advice on this subject is welcome.

I have used Econokote (same brand as Monokote) and I believe there is at least one other that uses low heat to stick.

Could you sheet the foam with balsa first and then cover it?  

I think most normal covering takes too much heat and the foam will melt.

Most normal covering will melt the foam, and sheeting it first can help but you still run the risk of melting the foam and creating a void under the sheeting. The low temp coverings work well as long as you get a good thermometer and set it at the lowest setting for the covering. It is best to start low and work up to a setting that will shrink the film and not melt a test piece of foam. The only problem with the low temp coverings is they don't work well with glow engines, it tends to desolve the backing over time.