Look at the proc "template::list::render_filters" in packages/acs-templating/tcl/list-procs.tcl.
This builds the multirow which contains info for each filter (including the URL) used by the "listfilters" tag.
The .adp page which actually emits the panel (after everything's calculated by "template::list::render_filters") is packages/acs-templating/resources/lists/filters.adp.
Thanks for the reply. That helped a lot to understand how it works. Now, thanks to you, Dave and Jeff I could manage to find a solution (temporarily at least) to my problem.