Hm, I'm writing documentation for my wrapper, and these little things crop up...
Anyways, StopAfterCurrent is incomplete, or rather it's missing a corresponding API call to cancel the stop-after-current request (result: MB keeps playing after the current file, as if StopAfterCurrent had never been called), a notification that the stop-after-current trigger is set and a getter call to find out if stop-after-current is scheduled - alternatively, a new PlayState flag.
I'm sure some desktop gadget might be able to use this productively...