Unfortunately, as you plainly understand, there\'s no way that a Scottish style boarded roof (sarking) can be vented as simply or in a manner similar to that of plain raftered roof arrangements.
The National Trust for Scotland and Historic Scotland, amongst others, have been and are still making a big mistake in encouraging the use of underfelt in the way that they do - it can only spell doom for many a doughty old Highland pile.
Unless sarking is allowed to breathe, it will eventually perish and much sooner than intended. Smothering this type of roof layout with any kind of underfelt, then perforating it with slate nails, is a recipe for certain disaster.
However, that being said, lapVent could be used if there was a slight change in approach and a rethink to the way that things are being done at the moment.