Surprised no-one answered you, NorthSouth.
First, you are correct that it is likely they will simply have asked for and received ATC clearance for crossing the airway. Usually that will be by the most direct route, with a height range quoted. These sailplanes are typically flying in wave, so may be able to climb as they pass across the airway rather than descend, depending on conditions. The one they can't generally do is hold altitude while crossing.
I can't speak for Inverness, having never asked to cross that corridor, but other ATC units have been helpful and accommodating in the past when direct crossings have been requested.
Some of the machines may have transponders; quite a number of high spec sailplanes do.
That airway is not particularly busy, in fact the whole Inverness area is generally not especially busy, so clearance should not be too hard to obtain. There are also high altitude 'wave boxes' which can be opened by ATC, by agreement with the various gliding clubs, in suitable conditions. I believe that normally requires twelve hours notice or more for activation.
I don't know how close to N560 those go, but they do mean that quite high altitude sailplanes might occasionally want to pass through N560.
I hope that helps.