Dumb Crank Question

I wouldn't want to comment on whether it is a good idea from a medical point-of-view - although if it really is only "occasionally" then you might decide to try it and see how it feels. Especially since you are reducing the crank length on the trainer. If you were proposing to increase the crank length beyond what you normally use I'd be very cautious as knee damage can result from over-extension of the joint.

However what you should think about is what you are trying to achieve by fitting the Stages. Whether it is to measure your power on the turbo - or to correlate an existing turbo power-output with the Stages PM ? In either case you may be fooling yourself. Even with a symmetrical set of cranks most people will have some small left/right bias in power-output. (If you're contemplating buying the Stages PM you've probably read some of the tomes written on the pros and cons of only measuring on one side). But whatever that bias is for you, my guess is that it could be well change if you use non-symmetric crank lengths.
So you can't assume the same L/R split on the trainer as you use on the road - which may defeat the purpose of fitting it.

I doubt you'll notice - I bust a crank and had to wait a bit for a 165mm replacement and rode around with a spare 170mm for a few weeks. No noticeable effect.

If it was purely for the turbo, then not a huge difference, BUT if it was going to be a long term idea, I would advise against it. unless you have one leg more than 2.5mm shorter than the other....

Seriously?! No, I don't think mixing crank lengths is a good idea - just invest in new cranks on the trainer.