If you are trying to attract entrants to events of less than 200 km then the usual suspects in central Scotland (with the wee enclave in Angus) will make up the bulk of your entry. Local cycling clubs are probably your next best bet if your event is early season.
Mid-summer, most club cyclists are racing up and down the country time-trialling, with a few road-racing. I think if you were to look at the entry for a 10, 25 or 50 mile time trial, the field would be quite acceptable in terms of size. A few with ambitions of MBAR and BAR will ride 100 milers and a 12 hr (for BAR), but I can only guess that this is a small percentage of active club cyclists. Scotland has, I believe, one 12 hr TT and even that is under threat.
I think there are a limited number of audaxers cyclists in Scotland who will ride more than 200km and for many reasons. If your event is 200km and above and the timing is right, a few of the BAR distance club cyclist may enter. I was fortunate a couple of times during the organisation of the Tayside Transgression when I received quite a few entrants from Stirling BC. They were either building up for the Scottish 12 hr, being encouraged by their mates, or lived very locally to the start!
A few times I have been fortunate to attract entrants from further afield. A couple were training for Le Etappe and came up from NE England and others have been holidaying in the area or had relatives who live close to the start.
So John M, I'm not sure. You solicited my opinion (and probably that of others) earlier in the year so the fact that you are posting here would suggest you do not have the solution you were perhaps looking for. Basically, I think it boils down to numbers and those who actively ride audaxes. In this part of the world these numbers are quite small. I don't know what the solution is either. I would love to have 50 plus riders on the Tayside Transgression 300km. Change the name to 'sportive' and charge £40 entry ?
However, does the success of an event present its own problems? If the size of the field for the Tayside was to grow to beyond 40, logistically, I would be struggling at the existing start/finish due to the car parking currently available. Would offering overnight accommodation attract more entrants ? It would also mean a significant increase to the entry fee to cover the cost of the Hall. Moving the Start / Finish somewhere else would be less attractive to me as an organiser in terms of distance from home to event HQ. Bringing the event further South would mean a major change to the route. Does that mean it's a different event entirely ?
So as others have suggested, flyers in local bike shops, and to local bike clubs. The CTC; local and neighbouring DAs. This forum (but those who read the audax pages are probable entrants any way).