Actually I had two lapses in thinking:
1. In the case of disconnecting the strap and using the rear channels for bi-amping, I neglected the fact that the two speaker halves each have a larger impedance while the available rail voltage has not increased. There may still be a marginal benefit but I doubt it's of any practical significance.
2. In the case of multi-channel connected to the fronts, the speaker impedance doesn't change but the rail voltage remains the same. More current is available but without voltage there is no "pressure" to push the current out.
However, if anyone has low impedance speakers (say a couple of Ohms), both of these ways would power them very well.
So I'm glad I drew that block diagram for myself