Last night (with some help) I cleaned out the threads, used teflon tape on 1/2 the threads and loctite on the other half, and tightened it down using a breaker bar with the Sheldon Brown tool. I'll put a dab of paint on tonight, although I suspect that any movement will lead to rapid unscrewing.

In the process I've learned that the crankarm extractor to fit my Stronglight cranks is unusual and pricey where available. Fortunately a friend has a Park Tools CCP-1, which fits but is no longer made.

Whoa! It may seem to fit, but that tool is actually designed for TA arms (23.0mm diameter extractor) while Stronglight is 23.35mm diameter extractor. You have a significant risk of stripping the extractor threads if you use the CCP-1 on a Stronglight arm. Sutherland's explicitly warns against this combination:

After reading far far too much online about the history of crank extractors late last night, I was beginning to suspect this was the case. Nonetheless, it did work.

I'm glad it worked for you this time, but seriously, don't expect it to always work.