I was just about to suggest mech-dump, good that you are already using it!

Mechanize will only return form elements that are within <form></form> elements.

The "All" dropdown is not within a set of form tags, it directly triggers javascript to reload the page. In cases like this you just have to figure out what the script is doing and duplicate. Possibly just by inspecting the request URL submitted by the browser.

This is an area where scraping pages becomes tedious and tricky.