doesn't that introduce some action-at-a-distance & maintenance difficulties in the far future?

I don't think so. At the point of logging, we know whether logging should be "once" or "always", and in all solutions proposed so far this would be clear from looking at the place where the logging occurs. From a programming point of view, I think a log_once() function would be easier to understand, though; OTOH, I expect that we might get diagnostics in the future (besides logging) which also should be executed only once, so I have not decided yet which way to go.

At least the discussion in this thread provided me with enough insight that I now see what possibilities I have.