GNOME fights VNC over display size

There's a bit of a usability issue when running GNOME inside a VNC X11 server:

If the user has at some point changed the display size in GNOME Settings, then GNOME will try to enforce that size on every login after that. This might be the right thing for a physical monitor, but it's not generally what VNC users want as they want the size of the VNC client window to control the session size.

A quick hack would be to look for "VNC-*" in the output name (I think other parts of GNOME does this), but that's not very robust. Perhaps there is some other mechanism we can implement in the VNC server to indicate that this is not a real monitor and should be mostly left alone?