How about this:
Periodically poll the PLM21's status with Girder. If the returned state is different from the perceived state, echo the state back to the PLM21. The command will be ignored by the PLM21 but will be seen by the M1's PSC05 and be used to update its record of the PLM21's status.
For example, the light was turned on via an X10 command and then turned off manually. So now both the M1 and Girder believe the light is on when it is actually off. Girder polls it, discovers it is off, transmits OFF to the light (ignored), the M1 sees the transmitted OFF and updates its records.
All of this is based on the assumption that the PLM21 is aware that the light was turned off manually and stores the new status! Otherwise, it will reply to a poll with the incorrect status.