For PA system-wide, suggest filing a bug. That something is not supported does not mean you cannot have an init script for it. Maybe it was done to discourage or to make clear that it is not the supported mode, no idea.
For multiseat, only this week there was a thread on systemd-devel that multiseat sometimes needs more configuration. I have 0 clue on multiseat, and don't understand what kind of configuration was needed, nor what you're doing with multiseat. Maybe they are targetting what you need, maybe not. Suggest writing to systemd-devel.