Chan, I think the filter route is the way to go, if I get nowhere looking for a low-light flash meter. Ideally, based on the other discussion, this would be a more "mechanical" filter (holes etc) than a normal ND, but I'm happy to work with Lee lighting gels in the meantime.

I wonder if common NDs, placed on a flash head, behave the same as when they are over a lens, in terms of reducing the output of a flash by a known log factor. I will check that at higher power levels with my meter.