Interesting idea, but I don't think it'd fix the softness. Here's why: As you may know, the foam doesn't provide any structural strength to the boat. It simply acts as a base over which to lay glass and epoxy. The weakness in soft hulls (I believe is that the layers of glass become separated from one another (and the foam). You can certainly remove the old foam, but adding new foam will not cause the layers of glass to re-laminate. The point of injecting epoxy is to have it fill in all the voids and thus provide compression strength to the structure, giving the layers of fiberglass something to "push against."
Even this is not as strong as the original laminated fiberglass layup where all the strength comes from the laminated fiberglass & epoxy.

Dunno if that made sense. Heh.