Ideally there should be a way for the regions to get on air should something like this happen. As I recall the regions used to handle GMTV playout (indeed, that's what got UTV in trouble back in the day when they broke in for some breaking news one morning). I'm guessing it wouldn't be simple for English and Welsh regions, though STV and UTV on that basis would be able to take control. However, with ITV in control of the franchise they wouldn't be able to without permission, and crashing in and out of a recorded programme
There's presumably nothing to stop them (ITV) inserting a regional news bulletin whenever they want during the standby programmes. I'd have thought it's no different to any other news bulletin, albeit cued from Chiswick rather than the studio.
All it would take is someone to organise it and get playout and the regions to agree a time. Though when the person who would organise/approve that is out in the pavement or at home I imagine it's difficult