I was going to pipe up here to add ReplicationSlotCtl to the list. Otherwise the only way to access slot information is via the SPI and pg_stat_replication_slots, which isn't super fun. And it's not like ReplicationSlotCtl is any more internal than MyReplicationSlot.
I missed the boat on your commit, but ... please?
Patches attached. MyReplicationSlot was only made PGDLLIMPORT in 9.6, so there's one for 9.4/9.5 and one for 9.6, 10, and master. Personally I don't care about 9.4/9.5 in the slightest for this, but that's where c572599c is backpatched to.