In flat calms, or nearly, we hook our tillerplot right to the wheel. Although on spec it's grossly underpowered for a 42-footer, it works just fine for powering in calms
. See http://www.sv-moira.com/projects.htm
and scroll down to the "autopilot" section (about half way down). When the wind comes up, the tillerpilot gets overpowered, which is a reminder to us that we really should be sailing anyhow.
You say "When there is no wind and you are under power, or in extremely light air, the combination autopilot/windvane might be the best possible way to achieve perfect self-steering with minimum power consumption." But if you're under power, your alternator is spinning, so what's the big deal with power consumption? If a vendor says "Don't do it," and you do it, then...on you own head be it.
"I still don't understand what course a tiller pilot is steering if not connected to the chartplotter or remote." The Simrad TP10 tillerpilot has a compass built in. Yes, you have to set it for the desired course. I'm not familiar with other brands.
In an earlier post you suggested that you could accomplish what you wanted by lashing the helm. That's certainly a low-cost, KISS alternative. Perhaps you could try that solution for a few years and see if it works for you?