There doesn't seem to be any direct relationship between modulus and loss: you can get high modulus (stiff) compounds with low loss, and then you get get softer ones with a low loss too. My guess is the problem with GreenGlue in a confined space such as you describe will be that it'll take years to dry as the water (solvent) can't escape easily - a 2 pack product would be better - as long as it doesn't shrink on curing. Might be worth getting in touch with Alchemie to see if they make a resin aimed at damping - they do a big range of PU products.