Thanks for the reply.

My first thought was to put a "set foo '-'" in the config, but when I use this pared-down example that emulates the documentation except to replace "content_by_lua" with "rewrite_by_lua":

https://gist.github.com/danslimmon/1ba367780f0efdd2afc5

I get "-" in the logs instead of "bar". Is it the "content"/"rewrite" difference?