Can any stereo amp be bridged to mono ?

Yes, it will.

In response to the title of your post, no, not any amplifier can be bridged.

No. Not any amp can be bridged. It depends on the design. For example, some amps are already internally bridged by design and cannot be bridged again.

If you are interesting in bridging to increase output, please check with the manufacturer.

Yes, but only if the amp uses a common ground.

Also remember that a bridged amp sees 1/2 of the original speaker resistance. If your speakers are 8 ohm the bridged amplifiers only see a 4 ohm load. Make sure your amps are stable at the reduced load.

Well, I beg to differ. Using the technique described (making one input out of phase) will work with any amplifier. The correct terminology is actually "invert polarity of one input", not "out of phase."

As for amps that are already bridged, I assume you mean balanced amps where the + and - speaker terminals are of opposite polarity. You are correct in the sense that nothing would be gained in terms of power, but it could be done.

The point about a common ground is valid. For instance, amps that are transformer coupled to the outputs and where the outputs float would need to have the - terminals tied together.

I am not suggesting that you should try this unless you know exactly what you are doing or that there will always be a benifit in doing so, but the answer to the original question is yes.