XLogInsert in my opinion is the best place to call it and the hook can be something like this "void xlog_insert_hook(NULL)" as all the throttling logic required is the current flush position which can be obtained from GetFlushRecPtr and the ReplicationSlotCtl. Attached a draft patch.
IMHO, it is not a good idea to call an external hook function inside a critical section. Generally, we ensure that we do not call any code path within a critical section which can throw an error and if we start calling the external hook then we lose that control. It should be blocked at the operation level itself e.g. ALTER TABLE READ ONLY, or by some other hook at a little higher level.
