Late last night I was able to finish up my 7AT wiring experiments. It was mostly a failure - there's no simple method (e.g. just cutting/splicing/grounding whatever one or more pins on the wiring to the Control Device) to force D to act it's in M without causing other problems. Had some fascinating partial failures though, including getting the "S" to light up over the dash gear indicator like S-mode in a 6MT
There probably is a way to do it, but you'd have to disassemble the control device itself and change some of the switches' wiring within it, rather than just doing simple work at the harness connector. I may or may not get around to playing with it anytime soon.
The basic problem is that the car doesn't like seeing the M-mode signal while in Park (or reverse, or neutral, etc). I was hoping it would just ignore it, but no such luck. Down inside the control device, of course, you could rewire the D<->M switch itself such that both sides are attached to the M side's wire and the D side's wire is unused. The switch is in series (electrically and also in a physical sense) with other controls inside the unit, which is why you can't just do the same thing out at the harness sanely.