Not a bad idea! Although, I think it'd be easier to just use some flat ABS sheet to create a frame/ribs inside and epoxy that in. That way its still somewhat flexible, so the inner structure won't end up coming through the outer skin.
Believe me, I've put some thought into this. :D I just don't know anything about working with ABS. I've even though about just using expansion foam from the hardware store, it wouldn't bond to the ABS but at least it would hold its shape and keep the spoiler from flexing as much. Or maybe some kind of combination of gluing in some ABS ribs and then filling each section with high density foam.
I may just end up using the fiberglass version, it'll probably take about the same amount of work to get it to fit right and look half decent, but I know how to work with fiberglass.
