The consensus seems to be that the Grenade Punch Maneuver could work on its target, given an attacker with the right blend of strength and toughness.
The main question now is whether or not (and how how much) the Grenade Punch would do to the attacker.
The physics/logic, though contested, seems to indicate this should seriously harm the attacker.
The preservation of game balance seems to indicate this should do as much (or more) damage to the attacker than an ordinary target of a grenade.
Issues of potentially bad precedent seem to suggest that this should seriously harm the attacker, or provide some other disincentive.
But Dresden has room for a little sketchiness in its physics. And the Toughness powers we are talking about are basically a form of Plot Armor. In the end, this may just have to fall out according to the spirit of the table.