For many of the journeys that you could make on the diverted Avanti services, the Routeing Guide maps already permit travel via the S&C.
The issue is that many walk-up fares have had silly "via X" restrictions added in recent years, e.g. Warrington to Carlisle fares are now routed "via Lancaster".
These via points have ostensibly been introduced to "simplify" the "complexity" of having fares routed Any Permitted. In reality they have done the precise opposite, since people now need to work out what intermediate stations their train passes through.
And in situations like this, it can cause problems by ruling out perfectly reasonable alternative routes - as demonstrated by the fact that it's a mapped route in the Routeing Guide!
Most advance fares are operator restricted (e.g. Avanti only) rather than geographically restricted so they're not affected by the same issues, but that obviously doesn't give you the ability to break your journey and you're tied to your booked train, so that probably wouldn't be suitable for what you mention.
In due course I'd expect an easement to be added to allow "via Lancaster" or "via Penrith" tickets to be used via Settle during the closures, but it's really poor that the most TOCs don't coordinate this in sufficient time to ensure diverted trains show up as valid from the moment they're published.