IBM Developerworks today has an article on "Making Dashboards with XQuery".
It uses eXist and provides a nice example of how to access external website data to create a mashup in XQuery. Source code is available as a starting point for your own experiments, though it is not optimized for speed in any way. Note: you may want to test this with eXist trunk, not 1.2.5 (processing in-memory fragments works much faster with trunk).
P.S.: also check Chris Wallace's article on "Dashboards and Widgets in XQuery"