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@moth
I had a similar problem, and am feeling pleased with myself for solving it, so thought i'd share. I too faced stripped non-drive side extractor thread, but with a miche sealed bb (detachable cups on both ends):
Remove the non-stripped DS crank, and the NDS crank bolt.
Pack the gap between the stripped crank and its bb cup with stuff - one of those cheap pressed steel multi-spanners worked for me, but one or more large cone spanners would probably work too.
Unscrew the DS cup about a third of the way.
Replace the DS crank. It should hit the DS cup while still loose on the taper: you may need to add more packing to make it loose enough.
Replace the DS crank bolt and tighten it to pull the axle out of the other crank.
Worked for me.
It may be better to remove the drive side bb cup if you have something suitable to hold the crank away from the frame, rather than applying all the force to the drive-side cup threads.
This should work fine with old non-cartridge, 2-cup bbs, and obviously will work the other way round if it's the drive side extractor threads that are stripped. For Shimano style 1-cup sealed bbs this is obviously useless for stripped drive side threads, but a variation might work for stripped non-drive side threads:
Remove the non-stripped DS crank, and the NDS crank bolt.
Unscrew the bb from the DS as far as you can, clean up and grease these threads, screw in and out a few times to work the grease in as far as you can - you're about to ask a lot of these threads, so you want them as well lubed as possible.
Screw the bb back in.
Pack the gap between the frame and the NDS crank.
Unscrew the bb from the DS to pull the axle out of the crank.
This method places the force on the DS bb threads and the bb bearings. You might be able to relieve the force on the bearing - i think a crank bolt through the middle of a cassette tool jammed into the middle of the Shimano bb tool might apply the pull directly to the axle, but that forces the axle to turn as you unscrew and still leaves the force on the DS bb threads.
The big risk with this method is that you wreck your bb shell threads and write off your frame. Probably more of a risk with aluminium frames than the steel one i was working on.