Here’s my work towards an Accessory Switch device type, maybe it could be of some help: https://gist.github.com/mckeed/182cc9689299e20af17b
It’s set up to allow either entering the device id of the target switch in the device preferences, or you could make a companion SmartApp that lets you select the accessory and target switch and just calls accessory.associate(target).
I’ve just been too super busy to test this and finish up proper support for these, sorry.