>could the "tail" on some cup and rings be an event marker – the position of a shadow cast by a wooden gnomon inserted into the central hole at a particular time of day and time of year?<

I've also thought along these lines. In effect the rock becomes a calendar or diary of events. Exact time and dates can be recorded according to where the shadow falls.

Sticking point is how would the length of the rod used to cast the shadow be kept constant over generations. Perhaps each rock or outcrop had a thin key stone that was kept onsite 'so to speak' which would always be used when the rock was incised.

Interesting hypothesis ,don't know how it could be tested though.

:-)