Never occurred to me, I must admit. The itemsx format is currently a straight translation of the old .items file format into xml.
As for adding cases ... the tricky bit is the expression parsing. It'd need to set up a parser that could handle logical and arithmetic expressions. We'd also need to grant at least read only access to a number of the games internal data structures.
All of which sounds like a script rather than a data file to me. So I'd say the way to go is to attach a script to the item (can't remember whether the triggers format currently supports that or not, but I think it does) and do the logic in the script.
I'm still planning on adding a Lua scripting engine to the game at some stage, so that might help as well.