Okay, that sort of worked (didn't realise you needed the '#' sign) - it created collections called 'no (read)' and 'yes (read)'. Thanks
I'd ideally like to use a custom column to translate the yes/no value to an 'read' and 'unread' tag, which I can then send over - that would probably be the neatest in terms of collections on the Reader. What template would I need to use to do that? (Sorry, I'm not terribly experienced with Calibre and this sort of thing).