Joel,
I'm not aware of any such property, at least that I can remember. My guess is that it's not there because that would mean the disassembler would need to hold on to all generated messages from the debatching until it read them all before it could return them (i.e. it would need to break the entire message and read it all the way though before returning a single message from GetNext()), which would completely hose up the streaming [goodness] nature of the debatching process.