They might be worth a try but there are better modern fry patterns nowadays such as the suspender minky for example. The thing is, these patterns are at their most effective when you can actually see fry feeding activity going on...apologies for stating the obvious.
Some of the more regular Loch Leven guys on here might call it differently (fire away guys) but I can think of only one season on Loch Leven when there was enough consistent 'surface' fry feeding activity going on to make it a viable tactic. That was back in '96 when salmon sized rainbows could be seen head and tailing, dying 6-8oz perch from off the surface.....and what a sight it was.....

I remember having no success at all with deer hair fry on those fish, though my friends and I caught several of them in the 5lb to 9lb class on big dries....however illogical that may sound. These were all rainbows though,no browns.
One thing in your favour is that there are far more weed-beds prevalent on the loch these days. These might just improve your chances of seeing some fry feeding, sticklebackers rather than perch though.Up till now most of the important features on Leven were of the unseen variety such as,drop offs,ledges and sunken shallows etc.