Can any multi-channel amp be bridged?

Eldartford, what you were doing with the center channel is not "bridging". In bridging (also called monoblocking, monobridging or summing) a stereo amp becomes a single-channel (mono) amp with much higher power (usually about 3X the rated power).

The setup you described with the third speaker is called a Hafler surround circuit. In a Hafler circuit, the third speaker is fed a left-minus-right signal. Anything that appears equally in both channels is cancelled, and what is left contains a lot of out-of-phase ambient information.

The Hafler circuit was used in the early surround-sound Quad systems with LP. I used one for quite a few years for movie sound, and it sounds fantastic with certain types of music. The Audio Research surround sound system is actually a Hafler circuit. There's a reviewer at The Absolute Sound who uses a Hafler setup for all his listening. NAD gives directions with their stereo amps for implementing a Hafler circuit, but it can be done with any stereo amp. All you need to do is connect the third speaker across the positive terminals of the stereo amp and connect to the positive and negative terminals of the speakers. Properly implemented, the ambient channel should be run at 10dB below the mains.

I've often thought I should go back to a Hafler system for movies. It has some advantages over Dolby and DTS.

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.