THE SAGA CONTINUES:

After putting 100 miles on the bike with this setup, I'm a little more reserved. It holds on well and I have no fear of the camera falling off, but there is readjustment with some frequency. I think, instead, I'm going to use a neoprene sleeve and a little daypack. It seems to be the simplest solution, and while I'm not crazy about riding in a daypack, it's also 2x as lightweight. It should do the trick.

The daypack I'm using is a Boreas Repack 15, which is very water resistant (siliconized nylon) so it should do the job. I'll have a plastic bag for the camera, of course.

Thanks all for the continuing help and advice.