I originated this thread a few years back and lo and behold, a bike appears on the market that incorporates my concept. The latest non e-bike version Hase Pino tandem has a regular bottom bracket and crank in a holder that fits the space where the Shimano STEPS motor is on the e-bike version.

I believe all of Shimano's current STEPS motors are using the newer, 8000 series mounting pattern. I do not know for certain for this to be fact.

As one respondent to the original thread pointed out, Shimano doesn't sell STEPS motors to non-builders, but the motors and other components are readily available in various markets.

Another respondent suggested a hub motor and that is certainly a reasonable direction to choose. However, the best supported e-bike systems are those from the big brands (Shimano, Bosch, Brose, Yamaha) with most aftermarket hub motors having inconsistently available dealer support. We've been burned once by the bankruptcy of an e-bike motor manufacturer (bionx) so this weighs into our thoughts going forward.

Choice is good, right?