I didn't realize they were made of aluminum. That does present some additional challenge.
Insulators at junctions and bonding rather than welding... maybe...
I could see using polyurethane bushings where the steel frame mounts to the reinforced points on the insight unibody. Then skinning it with aluminum sheeting that could be riveted to the insight panels. Composite/poly brackets could be bonded/riveted to the aluminum paneling to attach it to the steel frame and allow it to be stressed for rigidity.
I agree about not wanting to mess up the frame... It'd be a shame to do this to anything but a beater, at least to start with.
@rmay, do you already have a first gen insight?