I've been out of the grade for four years now and I've responded to the appeal. Normally, I'd say that it would be a completely unacceptable course of action, but these aren't normal times. I hope that things don't become so perilous that they have to call upon those who have responded, but I think it'd be irresponsible if they didn't develop a contingency plan in case staffing levels do reach a level where they're struggling to keep even the most important routes open, because if that happens (with the obvious impact on the supply chain) then we're all in trouble.
I'm surprised that it's extended to anyone with heritage experience alone, but again I guess that it'd be foolish to rule anything out at this stage, and there are definitely some decent candidates - the problem being those who don't know their own limits and think that working a single line heritage box and a bit of Simsig means that they're good to be thrown straight onto the station panel at New Street! I've seen a proposal that suggests three levels of 'contingency signaller', ranging from (IIRC) an 'assistant' in a ROC or a multi-manned panel to someone working a lower-grade box alone. Personally, with a bit of refresher training, I'd be comfortable working any of my former boxes if it came to it, certainly as an alternative to shutting the line altogether.
I understand the union's concerns. They're absolutely right to quietly oppose it. The last thing that anyone wants to see is a long-term contingency workforce created. For now, though, I'd hope that everyone would co-operate to make sure that we can keep the job moving, because we have no idea how bad the situation will be in a couple of weeks.