Datasheet: https://www.mouser.com/ds/2/137/epson_1705528-1165749.pdf

They're rated for a maximum drive of 1µW. I'm trying to relate that to maximum rms voltage across the crystal. Looking for some sort of effective resistance at resonance, I find the only resistance value on the data sheet to be the motional resistance. Is this the number that I need? For my crystal, the motional resistance is 35k. Using the P=v^2/r formula, I get a maximum rms voltage across the crystal of 0.187 volts. Does that sound right?

I'd been thinking of using inverse parallel diodes across the crystal to protect it, but even Schottkys would have too high a forward voltage for this. Plus, the specified motional resistance is a maximum value. If the motional resistance is smaller, then the maximum voltage would also have to decrease. This implies that I should be limiting current rather than voltage, but that could be a problem in the circuit that I have in mind.

FYI: Yes I'm aware of several circuits specifically designed for these crystals, but I need to use these in a vacuum tube oscillator, which makes life more interesting.