The economics of the Far North line are challenging no matter how you put it.
Adding an additional carriage for LEJOG cyclists just seems a bit silly - it’d benefit a tiny number of cyclists who have plenty of good alternative options, with a massive fuel penalty. That money might be better spent adding more cycling carriages to trains to the Clyde coast - which frequently carry a dozen bikes on summer weekends.
If we’re in the mood to spend money, why not give Wick a handful of 153s and its own tiny depot? You could run a shuttle service to avoid the detour via Thurso, and also maybe some local services.
Or, do something radical like freeing up capacity for railway preservationists to use? You could turn the line into some kind of massive preserved railway, substituting some of the regular Thurso and Wick trains for steam trains over the summer, while still providing a full passenger service.