Thanks for your reply Chandra!
I saw that thread while looking for the right solution - but it doesn't really answer my question. The options mentioned there are basically DOM fiddling, which is exactly what I don't want to do.
What I'm looking for is a way to tell the OData model to always prepend an "all" option, and not leave it to the dropdown control to take care of that. Or, alternatively, the information that this is not possible 
Cheers,
Christoph
