Suggested by Giles
This does require the user to know the types are complex to start with, and then to figure out what you just explained [about the XMLSplitters] . Though I did see an "XMLsplitter" option when I right clicked, it did not occur to me that that might be the thing to explore, even though I had a fair idea of why the parameters were being not being listed. An end-user, discovering a service through Feta for example, is even less likely to (though the Feta description might help the user figure it out).
A big, clear indication when you mouseover like "This parameter is complex, and made up of lots of little parameters. Would you like me to expand it?" might help.
Doing something like this should not be too difficult.