I know the following won’t fit a lot of these cases, and I’m not sure if it will fit yours, but I did want to mention that there is now a battery operated switch cover which is bistable because it physically moves the switch underneath, rather than being wired in to the mains.
If you don’t want it to be battery operated you would have to modify the power supply. (The good news is that it runs on two AA batteries, so it should be easy to use dummy batteries with a plug-in pack instead.)
It’s intended for light switches and there is both a toggle version and a rocker version.
So for someone who happens to have a Device which could be wired to a regular toggle switch you could then fit this zwave device over the top of it, giving you a bi-stable Z wave controllable set up. And you don’t have to worry about matching voltage as it’s just going to physically move the switch underneath. 
So again, not a match for every use case of this type, but it just came on the market last year and I thought it was worth mentioning.
