> continuously variable phase control into a preamp? Sometimes it seems a mere in/out switch isn't enough.
Then you probably want a delay line, to equalize the air path length among several mikes all hearing the same source. Moving the mike is by far the cheapest and most-correct answer, but sometimes that conflicts with other choices. Digital delay lines are now not-crappy, and may be better than fighting phases.
> getting equal shift at all frequencies.
Not the same thing at all. A 1 foot path difference will be nearly zero phase at 50Hz, but many cycles out of phase at 15KHz.
> physically impossible, right?
I don't think there is an elegant way to get arbitary phase shift, constant over frequency, on complex waveforms.
But in real life, you cascade several phase shifters at overlapping frequencies. The number of phas shifters is determined by how wide a frequency range you need and how far off the mark you can let the phase get. SSB radio transmitters used to use "90 degree" phase shifters, which were really +/-10 degrees 500Hz-3KHz. That was good enough for voice communications. They were not simple, and even DIY radiomen often bought a canned phase-shift network. Op-amps make it easier, but trying to get 5 degree precision over hi-fi bandwidth probably takes an awful lot of parts, fairly well matched, and potential smearing from many buffer amplifiers.