I don't have a lot of experience but it's entirely possible they'd just tell you not to worry about it if you try and get one, when I went up to Fort William in the seated coach then lots of people got on who didn't have reservations and the guard didn't care in the slightest, it was just like any other daytime service. I had a reservation to FTW but there was no way for anyone to check whether or not I had one so I got the impression that it doesn't matter.
Of course a larger problem arises if they start telling passengers joining the seated coach that they need a reservation, and reservations are still as hard to get as they are now.