That's not an emergency. So no it's not imo a valid use of the passcom.
If the train had already started it's a bit pointless as the outcome will just be to carry on and go back, but would consider it valid to use it while the train was still stationary if a failed door release had obviously occurred with insufficient time to get to a working door, and definitely valid in the case of a total fail-to-call (i.e. didn't stop at all and no announcement) as it could indicate the driver being unwell. I've used it this way (and then the egress because nobody answered it) on a Thameslink at Blackfriars where it had terminated there, seemingly the driver had pressed the release, noticed people stopping alighting, pressed the close and walked off, but it seemed the release had not actually worked in every coach so we couldn't open the doors. Other option was to sleep on there!
Clearly demonstrates why the use of porter buttons* to check each coach is the right way to do it.
The other door related one I would use it for is a separated parent and young child, regardless of why it happened, as that is a potential risk to life/health of the child.
I also used it once (while still stationary) when they forgot about a cyclist on a Pendolino at Brum Intl and staff didn't see me shouting and waving at them, where due to the time of day and diversions being on he would have got stranded at MKC if he had been overcarried (and that's a long and hilly cycle ride in the dark potentially without lights). The guard sneered at me a bit (no doubt because he had to walk pretty much the length of the 11-car to release the cyclist and reset the passcom) but did resolve the situation. Similarly I'd strongly consider using it in the event of the failure to provide assistance to a wheelchair user, particularly if they were distressed, though when I've had that happen waving at the guard with my foot in the door has generally worked.
Other time was a father and son on the Tube where the father was clearly unwell (looked like some sort of low grade fit) and the son panicking, again pulled at a station (Victoria, if I recall) to attract the staff's attention. Opinion was split in the coach as to whether it was the right thing to do (as opposed to helping them off and allowing the train to continue) but the staff who attended thought it was.
* 700s don't have physical ones but do have a mode where the normal door buttons can be used to achieve the same thing.