All necessary stuff is available in the systemd overlay.
Created attachment 277195 [details]
One-interface unit for wpa_supplicant (wpa_supplicant@.service)
As I mentioned earlier, that service file is useful only to NetworkManager, and I'm not even sure if NM is not a better place to install it.
I'm attaching a simple unit of mine which allows running wpa_supplicant on a single interface (without NM).
CC-ing NetworkManager maintainers too to see what they think on #c1.
(In reply to comment #1)
> Created attachment 277195 [details]
> One-interface unit for wpa_supplicant (wpa_supplicant@.service)
I would suggest changing
Description=WPA supplicant daemon (interface-specific version)
to something like
Description=WPA supplicant daemon on %I
Ok. Added -r3 to tree with systemd unit