Isn't the problem that the only person you can show discretion is the TM‡ of the later train. ...
I suspect you could get it endorsed as "valid on next train" by the conductor on the one with the missing Pulman
Slight problem though, if I intend to board at Taunton and observe the absence of the advertised Pullman as the train pulls in, I rather doubt that I could locate the conductor and ask them to endorse my ticket before the train leaves. With me either on the service without the Pullman, or left on the platform without having had the ticket endorsed.