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Do any manufacturers (either of old-timey tubes or new tubes or amps)
offer specific numeric limits on allowable grid current in a beam
tetrode in class A or AB1? I'm thinking specifically of tubes in the
6L6GC/807/6146 class.

I'm finding that the best of my older tubes have a grid current of a
little less than a microamp when biased at -20 or -25V on the grid,
250V or 300V on the screen, and 500V or 600V on the plate for a nominal
plate current of 30 or 40mA, most are in the low microamp range, and
some skyrocket into the hundreds of microamps or even milliamps within
minutes.

It's possible that the bad/worst ones would be perfectly acceptable in
a transformer-input class B modulator/audio amp or in a class C RF amp.

Tim.