Hi @sjaak, success! My colleague @Ramesh_Natarajan_RN has pointed out that you can actually get the message count from the monitor API:
GET http://localhost:8080/SEMP/v2/monitor/msgVpns/default/queues/<Queue Name>
yields
"collections":{
"msgs":{
"count":1
},
"priorities":{},
...
You can also query all the queues in your MessageVPN and get the message count for each. Thanks @Ramesh_Natarajan_RN !