I don't think the smooth surface layer of the dynamat is good for direct absorption, there's a reason why direct acoustic damping material tend always to be extremely porous. Maybe if you can find dynamat without the metallic surface layer it'd work. Vibration damping is a whole different thing here.
If you don't believe me, look into popular damping mods (e.g. T50RP mods, Denon D2/5/7k mods), dynamat is always applied to the back and around the drivers, while porous damping material (whether it be foam, felt, or something similar) is applied on the inner faces of the drivers. They serve different functions.
Edited by jerg - 11/19/13 at 10:24am