I haven't seen anything that shows it *can't* be used, especially if the middrive has a ramp-up to power rather than slamming it on. Then you can also use the throttle (if you have one, rather than PAS) to ramp it up slowly, too.
FWIW, if you don't need as big a gearing range and don't mind the step size, you can use a 3 speed instead. I've had three different brands and at least two different kinds of 3 speeds, and they all have a 1:1 direct drive gear, where they basically just lock the input shaft to the shell (so it's strength limit isn't in the gears themselves in that ratio).
Mine are all older types, but newer ones probably do too.
And being less gears, but probably the same width, the other two gears are probably stronger by at least a little, than one with narrower gears/more of them. That's an assumption that may be completely unfounded, however, so you'd have to check into that.
Also, I have not yet run anything other than leg power thru any of them, though the plan is to eventually do so.
I *do* have the trikes' pedal drivetrain geared way down so at lowest gear it can pull the heavily loaded trike along, albeit at a very very low speed, in pedal only, even with my bad joints, for at least short distances between rests. Just in case something happens to the power system.
