I actually tried exactly what your are describing about a year ago. It works well, the tips I would give are to use a rubber "kickball" and cut just one slit for the nozzle of greatstuff. Before you fill it, run some water inside, swish it around and drain it out. For whatever reason, I've found the residual dampness from the water helps it to expand and cure better. You won't need too much, just moderate amount and leave it for a few hours, it will graudally expand.
I'd also recommend buying the lower expanding, denser type greatstuff sold for doorframes. Don't leave it out in the sun for a year, or the rubber will start to dry out and crack apart.
It can be a lot of fun to kick it up a hill and shoot it as it turns around and comes back down, or give it a nudge from the top of a hill and send it on it's way. Blunts may bounce off though. I use field tips.