No luck here rspring ~ I'm thinking this is not possible, or at least not documented.
For the short term, I have been able to (mostly) workaround by creating an island table with one field containing just about every possible value that could be passed in, and using the select=ListBoxID syntax. The list of values comes from the source system where the URL is being generated, but values can be added all day long. So in my case this wouldn't handle new values added in the source system (why I wanted to use a variable to begin with.) Not optimal, but I guess it'll have to do for now. Then I use a trigger on the defined field that attempts to select the passed in value in my "real" field.