Are you meaning getting off a rail replacement bus / coach at a scheduled stop short of where your ticket is valid to? If so, I doubt it would be an issue - rail replacement drivers and controllers are generally not expected to check tickets in detail - they migth check that passengers have a ticket before getting on, but they won't check tickets of people getting off, and you won't have to go through the ticket barriers. (seeing revenue protection staff round rail replacement services can happen, but it's a rarity.)
If however you mean getting off at somewhere the bus / coach isn't scheduled to stop...
There will be a set route for rail replacement buses / coaches, but it's not unknown for drivers to find their own way either because they think they know better, or they are aware of traffic congestion somewhere, or because they get lost. (And for that matter, if it's a rail replacement route that happens regularly, it's not unknown for the 'official' route to have been changed, but some drivers to follow the old official route.)
Unofficial stopping points are, as others have said, officially not made, but unofficially can sometimes be at the driver's discretion. I don't know that part of the world at all, so can't comment if there might even be somewhere safe to stop - would coach have to go through town centre or is there a by-pass or something?
The risks to the driver of stopping somewhere unofficial is that if the passenger has any sort of accident getting off, it's going to result in awkward questions, and if driver says 'yes' to one passenger and 'no' to another (even if the first location is safe and convenient and the second isn't), the disgruntled passenger/s might put in a complaint.
And if the passenger wanting an unofficial stop has got luggage in a boot / side locker of the coach, the answer will almost certainly be no.