Thought I'd wrap up.
I was again possessed by this thought, and had a go now that we have a small set of ccr2004's around. No dice. No warning while I was entering the config, but when I did an export after the fact, handy little note:
/interface bridge port-controller
# no hw support
Whole thing makes sense. It'd be something of a custom implementation to let the CCR2004 do it.