The way I would go about it is to use the update_html function to tweak the HTML currently held by Mechanize until it matches what the website expects, or at least contains the one value you need selected. That's what that function is there for after all.

Mind you I'm a bit baffled as to what that JavaScript does, out of context, That setTrim function does something to the code of the select menu in the DOM as soon as you make a selection, and your snippet doesn't cover it.

So, another FireFox solution, like kwapings only with one more layer -- you install the LiveHTTPHeaders extension, fill out the form and see what actual POST and/or GET values get sent by the browser, not what the form says?