Well AFAIK its section 129.2 Roads Scotland Act 1984 (obstruction of traffic) and possibly also 129.6 blocking a cycle route. The English (s.137 Highways Act 1980 - split away from the original s.72 HA 1835) equivalent is a level 3 tariff which has a maximum fine of £1000.
Unlike driving on the footway there is no in flagrante delicto requirement so a photograph (or repeated daily pictures) should emphasise the case and demand action from Police Scotland.
Then again if there is one of those fine heavy duty bakfiets (the sort for carrying a piano) around and a suitably decent load I'd fancy piloting it along the road with as much of the bakfiets as possible in the cycle lane (it would probably just about reach the rear wheel...)
Although the low geared fixed wheel versions I've ridden seem reasonably good at stopping I'm told that the braking is not actually that good with a load on.....