It was suggested I post this to tor-dev; I prototyped a small thing that allows the prop-279 APIs to be tested against actual-Tor without changing Tor. That is, it does the "abuse the control-protocol" thing but speaks to prop-279 style subprocesses.
This is just proof-of-concept and would need a little more fleshing out, but should be sufficient to proceed with prototype implementations of prop-279 clients.
https://github.com/meejah/TorNS