I'd be using most likely the same diameter pipe (basically the same as the internal shoulder size, just big enough to fit inside the existing cover bearing IDs if possible, and pass thru any freewheel or disc rotor normally).
I'd rather not drill both sides, and prefer only to do one (and that really only to hold the removable half of the clamp to it for ease of install, with the side effect of helping resist torque).
So if that kind of setup would not have troulbe with the torque levels I have (or want) on the SB Cruiser trike, I'd rather stick to just the one screw not protruding into the pipe ID at all, if possible.
I don't know the Nm numbers, but assume a max of 3/4 ton of trike, trailer, cargo, all at once, with enough torque to acclerate that from a stop on whatever uphill slope is typical for heading into an intersection on "flat" ground to 20mph in <3 seconds. ~4 seconds if the motor/controller combo I end up with eventually can't handle sub-3.
The trike full of groceries, plus me, is probably about 600lbs, and that's the most common situation I'd need that accleration with, but very occasionally I have a trailer full of heavy cargo to move (once every year or two) and I don't want things to break then, either.
I don't need high speeds, but I do need it to get to 20mph as quickly as possible without breaking anything (including tire traction), and I also need it to be able to handle regen torque on those wheels stopping that kind of mass as fast as possible, too (in case there is a mechanical disc brake failure, etc), wihtout breaking the axle clamping force and allowing twisting of the pipe.
Right now I already know the axle flats (of a pair of MXUS 3ks in 20" wheels), even in a clamp that secures the *entire length* of the flatted section for the inboard ends, is not sufficient to completely prevent axle problems, anything from eventual loosening over time to actual axle twisting or even breaking at the axle shoulder (transition from flatted to round).
The motors I will probably use for this pipe-axle mod are a pair of QS205s (I only have one complete one, the other is just a stator, no rotor or side covers, not sure what I'll do about that).
(I also have one UltraMotor from a stromer, gutted and wired for external controller; it's axle is MUCH better than the MXUS but it's still the typical flatted-sides type and has the same weakness in design).