Accounts service now provides the XSession property for users. It would be nice if it migrated the existing user session from ~/.dmrc.
I think the behaviour just needs to be:
On XSession query, check the value from ~/.dmrc, or if this does not exist /var/cache/gdm/user/dmrc.
The major complication with this is that accountsservice runs as root, and root shouldn't be looking inside users home directory. That's actually one of the main reasons Matthias made accountsservice initially: to have a local place where root could access user data.
I'm going to close this wontfix, but feel free to comment if you want to talk about it more and we can reopen if necessary.