Yup, this is the real story. Find the high voltage capacitor, run wires out for + and -, and run any voltage in up to 140V. I'd suggest using at least 24V because of the way they might be bootstrapping the high drive.
The control electronics still has to be powered. The MSW inverter must have an on off switch, not momentary. Locate the side that doesn't go to the 12V terminal. Supply 12V to that side and it will require around 50ma. As long as that voltage stays in reason, the inverter will keep running. I ran a 50W 24V lamp on 20V.
This has possibilities. Many small AC motors are impedance protected so they can be browned out a bit. With a 60V array, you could use a boost transformer to get 120V and run 90-130V for example. Still must be some considerations for turning it on and off at low voltage