Oban isn't exactly handy for Fort William. Could see the benefit of running as far as Crianlarich though then rail replacement coaches from there on.
Crianlarich-Oban is RA5, so a 67 (or 47) would be too heavy.
In the past the standard diversion for the Fort William sleeper has been to Oban if there's a blockage north of Crianlarich.
It might not seem convenient, but the coach journey time from Oban to Fort William is around the same as Crianlarich to Fort William - and a fair number of the passengers are coming from further afield, often by car or ferry, so it might actually be more convenient to join at Oban.
Having the sleeper diverted to Oban also means you get an extra hour in bed compared to changing to a coach at Crianlarich, although the total journey time is longer. On a day train it would be a different matter, and a coach from Crianlarich would be the preferred option for most people.
Can't they borrow a Deltic from someone - I'm sure those have gone to Oban before!