This is a feature request.
It would be very useful from a 'what the heck is my system doing' point of view to know how many operations make up each outstanding SYNC request.
A system that is behind by 1000 SYNC's with no row edits in any of them is in a very different state than a system that is 1000 SYNC's behind each with 10,000 UPDATE statements.
Similarly if the next/current SYNC to be processed consists of a million rows that might explain why replication 'seems' to have stopped from an sl_status point of view.
The test_slony_state.pl script seems like the most logical place to add this though other ideas are welcome.
The logic for this is liable to need a stored function (or similar) which duplicates the logic in remote_worker.c which associates XIDs with SYNCs.
If we implement this (which could be quite a good idea!), this might provide a way to shift the SYNC contents evaluation out of sync_event() (in src/slon/remote_worker.c) into (a) stored function(s).
It's definitely nontrivial, but, if done, would pull a pretty complicated piece out of the C code.