It's -1 for all three. I sort of assumed "SS.MFOn" might not exist. Do you think its worth "cave diving" into the strings.txt file for something closer? Even if that exists for one of the cameras, it still eludes the goal of a universal way to enable MF.
On D10, these function appear to be called PT_MFOn and PT_MFOff, registered by SS.Create. These seem to work as c10ud described. The MF indicator does not appear, but set_focus takes effect immediately and no AF scan is done on half shoot.
It would be interesting to know if this works on cameras that do not natively have MF. However, to test this with set_focus, you'd probably need to change the camera.h defines to be like camera that does have MF.
We may not have a universal method, but having options to try on different cameras is better than nothing.
These evenprocs probably only exist under a handful of names, and it would be fairly easy to find out which ones use which from the dumps.
edit:
On A540
EnterToManualFocus disables the canon AF scan, but set_focus doesn't seem to work, either immediately or on half press. However, if I force the FOCUS_MODE propcase to 1, it does work immediately.