Right, and why has this not published in writing anywhere or actually been formally organised by the TOC that's unable to run any services? This is the crux of the point I'm trying to make (no, I'm not travelling myself).
Frankly it's not good enough to tell customers "You might be able to travel via this route, if the train manager is pragmatic and understands the situation you're in, you should ask the guard before boarding" nor "Try your luck contacting the TOC via Twitter, we don't know the answer" nor "Oh, by the way, if it's your unlucky day you might actually be charged a change of route excess, sorry" if you're a retailer. There's generally an expectation from customers that the industry has put contingency plans in place and that, as a retailer, you understand the current state of affairs and can give them a concrete answer as to their ticket validity. Currently there's nothing concrete, so everyone just gets a refund even if they still want to try and travel.
You and I know that the forums are a good place to ask for advice on what to do here for customers "in the know" - and the advice given is good. But most customers aren't in the know, and will be wary of travelling off-route without actual confirmation that they can from an actual authority.
Yes - I think there is a clear case for such an agreement to be put in place. That does not appear to have happened yet from the conversation I had earlier with Chiltern's customer services staff.