That could make it louder. I've built my own set of DML panel speakers, and coating the outside of the panels in a coat of PVA glue enhanced the high frequency sound. If there's good connection between the epoxy and the surface, and the exciter is powerful enough to energize it with the extra mass of the epoxy, it will still work. Some people even coat their panels in expoy to try enhance the sound in a similar manner to coating in PVA glue. You would have to get the epoxy jammed in on the side with the exciter, which is usually not accessible.

it would probably fuck up the resonance and bring the level down at least

Exactly where my head went, but with expanding spray foam to get in and out quickly. Don't know if that would even defeat these panels though.