I would recommend capturing the user input using XForms, and then using XSLT
to take the XForms instance as input and generate (or parameterize) the
XQuery as output. Then you can string the whole lot together using something
like Orbeon, which provides both an XForms processor and an XProc-like
pipeline processing language.

xforms are on the todo list, so I can't appreciate what you are
suggesting there but it does sound a bit heavy.

--
Dave Pawson
XSLT XSL-FO FAQ.
http://www.dpawson.co.uk