Tagging all traffic from port or interface with VLAN ID 0

How could you tag all (egress) traffic from a port or interface with VLAN ID 0 in RouterOS?

This is a general question, i.e. I am not trying to solve any specific problem. I am also aware this is non-standard. However, the reason I ask is because some manufacturers and service providers DO use VLAN ID 0 in a non-standard way, including major ISPs like US AT&T. At some point this could be an issue for anyone and, of course, out of technical curiosity.

I already tried to find out how to do this (to send priority-only frames). Maybe it’s not implemented.
Would be worth a feature-request. Strange VLAN-ID=1 problems may also be addressed with this change.