It is my understanding that we do not require any special declaration to use the cups-socket-directory attribute so this should work as soon as it is added. @till.kamppeter could you please check and let us know if you experience any issue?
I am +1 as well for for auto-connect the cups slot. +2 votes for, 0 votes against. Granting auto-connect of the cups slot to the cups snap. This is now live.