Can any multi-channel amp be bridged?

Audiobomber...Also called the Dynaco matrix multichannel setup, and they sold hardware to facilitate its setup. They derived a center front channel by using a Y connection of the front Left and Right with the return through the center. What I did was to make the fronts the back by reversing the phase of one stereo channel. If I played a monaural source, my setup would be exactly the same as "bridging". The center was driven by the differential amp outputs, with the two front speakers driven by the single ended outputs. I preferred my setup to the Dynaco Y-connected center because I don't like the idea of putting speakers in series.
Other advantages were easy control of the center channel volume (all it takes is a "blend" pot), and easy derivation of a low level "rear" signal, Left plus Right, (plus because one signal is inverted) so that the rears can be driven with a separate amp. The only difficulty was getting the phase inversion for other sources, like FM Tuner.

easy derivation of a low level "rear" signal, Left plus Right, (plus because one signal is inverted) so that the rears can be driven with a separate amp

I see you've heard of the Hafler/Dynaco circuit. Great stuff! But I'm quite certain the Hafler movie surround setup is L+R for the front center channel and L-R for the rears. Just like Dolby Pro-Logic, but without the phasey "steering" that craps up DPL sound.

I disagree that your experiment had anything to do with bridging, since there is only one single channel in a bridged stereo amp. It's a totally unrelated concept.

Audiobomber...Of course the required center speaker signal is L+R, but there is more than one way to make this happen. One way is to mix the low level L and R signals, and drive a separate amp. Another way is to make a Y connection of the three front speakers, so that the returns from L and R are tied together and then connected to the High terminal of the center speaker, whose Low goes to the Amp "common". My way was to invert the phase of one channel (eg: Right) and then to bridge the amp with the center speaker. In this case the signal that the center speaker receives is (L-(-R)), which of course is L+R.

Think about it some more and draw a picture (again I wish that the Audiogon site had some way to include a diagram in a post). My setup actually resulted in a lower impedance load on the amp than mono bridging, because in addition to the center speaker across the two amps, each amp was also loaded with a speaker, L or R. But, somehow I never had a problem.

Back in the very early days of matrix multichannel I did a lot of experimentation, and discovered lots of interesting things that never got incorporated in commercially marketed equipment. I even created my own logic-assisted system before these were on the market, using a DBX expander/compresser. Most of the problems with logic-assisted matrix systems can be overcome if the process is digitally implemented.