Hi, Olaf. Unfortunately I don't have any suggestion for this since I'm looking for the answer myself. However, you should check out (ie, vote for) an "idea" that's already been submitted for something like this http://goo.gl/ANnK2E. It's been sitting with SAP for almost a year now which is unfortunate.
I'm using the SAPCallMemberSelector api which does nothing until the query is refreshed. However, if you use the "prompts" dialog the member selector works without first requiring a refresh. No reason why the API can't function the same. Guess all we can do is vote and pray.