I have not. I have seen other electrified beach cruisers.
Mounting a hub motor should be pretty straight-forward.
Mid mount motor will require a BB adaptor and you will likely run into other problems.
Bottle battery pack can be mounted to the seat tube.
One problem: Hub motors do not have coaster brakes.
You will have to add hand brakes unless you choose a front hub motor.
Disk brakes on a Cranbrook will involve a lot of parts, adaptors and money, IMHO.
Easier to start with a disc equipped bike.
You could mount caliper brakes. I added them front and rear on a Cranbrook I converted to a 10 speed. Not as good as discs, but easier to do. If you keep the speed within the same range as a regular bike you'd be okay.
The Cranbrook that I am familiar with has a 1pc crank. If you want pedal assist , the standard sensor is for left side of a 3pc crank spindle. There is an aftermarket right side magnet ( mount magnets on back of chain wheel) and sensor that requires DIY install that can be used with the 1pc crank. Otherwise , use a 3pc conversion kit with a long enough spindle for the left side magnet ring to fit between the crank arm and bottom bracket.
The Cranbrook has a 110mm rear hub width. Is there any wheel with a cassette that would fit into that tiny space?
Alas, everything I've seen is for a 130mm hub width...
So far the thought is to drop in a TSDZ2 mid drive into the bb with eccentric adapters, and put on a front springer with disc brakes. Assuming I can find a back wheel with a cassette, probably vintage, I could do an ebike with front disc brakes only. Is that a bad thing?
I went and got a frame that already had a 135mm rear spread... And it already had a freewheel and v brakes too... Working on the "e" portion of the ebike now...