Seems to me it would easier to just device a 3-speed hub design for the motor drive sprocket that would gear it say 1/2 a tooth either way internally.
You would need a modified clutch cover but the sprockets and chain themselves would remain aligned.
And for the 'dreaded tensioner problem', you almost Never Need a Drive Chain Tensioner!
And for 410H Z chain I'm sold.
The regular Z chain my Giant 3-speed shifter came stock with and the jackshaft I put on broke it after about 100 miles.
It now sports the heavy duty 410H and no problems.
Just more ideas to keep a coaster pristine but add gears to the second drive chain ;-}