baker transmissions and champion trike both make reverse gear sets for hardleys. they are extensive takedown and rebuild kits for hardley transmissions and use/require the motor and clutch to do reverse work. its a fair piece cumbersome and wrought with all kinds of potential problems including nuetral interlocks. Champions version is notorious for hosing a tranny if you put it in 1st gear w/o disengaging Reverse.
Hardleys have em cuz they are subject to trike work for muey many years. the Yaps(j) cept for the Leadwing, are newcomers to the trike world. 'Lectric reverse seems to be the more better way to get er dun.
If yor looking for ideas as to HOW they accomplish that, do some research in the world of trikes and you will uncover all kinds of bizarro methods. somes are ok, somes are ? Wether those methods can be ported over to a skoot is probly why they haven't... and there isn't much demand/market for it.? ponch out