I don't believe they do. A hundred or so pages ago someone posted a photo of a walk-up CS ticket which was issued as a till receipt from the lounge car's POS. Can't see other TOCs accepting that.
I’ve never tried to buy a walk-up CS ticket, but I seem to remember that they are sold by the hosts, rather than the guards (at least south of Edinburgh), as the guards who work between Edinburgh and London are non-commercial if I remember correctly (i.e. not revenue trained).
I’m not sure about the status of the guards who work north of Edinburgh on the Highlander, as these replaced Scotrail conductors who were revenue trained and could, of course, sell tickets using their Scotrail-issued ticket machines. I would have thought that the guards who work to Inverness and Fort William must be revenue trained, as they are required to sell tickets to day passengers from paytrain stations (I.e. between Helensburgh and Fort William and also from Carrbridge on the HML, which has no ticket issuing facilities).
Quite why CS don’t use conventional machines which print conventional ticket stock is bizarre, especially as Inverness is gated (though the barriers for platforms 1 and 2 are nearly always left open when the sleeper arrives in the morning). I tend to get off at Spean Bridge rather than Fort William; there was some talk (perhaps nothing more than messroom speculation) of installing barriers at Fort William a while back but I don’t think that ever got anywhere; certainly none were there last September, the last time I used Fort William itself.
Another thing I’m wondering about (as this is a journey I might be doing myself sometime)- if I get onto the sleeper at Carrbridge at 07:55, to connect with the 08:55 Inverness to Kyle train at Inverness, what’s the chance that the CS guard would be able to sell a through ticket? Carrbridge is unstaffed and has no ticket issuing facilities, so there’s no chance to buy before boarding. Technically CS ought to be able to sell a through ticket as it’s a national rail service, but I’m not sure what the situation is here.