You can do anything. But you must include custom code.
Untested example:
<button type="submit" onmousedown="document.searchForm.elements['gs[FIELD][]'].value=''">clear</button>
FIELD is the name of your Groupselect-Box. (e.g. category for the example database on this site)
If the button is pressed it should set the value of the select to empty and submit the form.
If you use a normal button it clears the select without submit.
This will not work for multiple selects. For multiple select you must enhance your code.
<button type="submit" onmousedown="Array.each(document.searchForm.elements['gs[FIELD][]'].options, function(item){item.selected=false});">clear</button>
Works with mootools only.
