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.