I think it would have to be AppleScript, and you should be able to an idea of how to do it from @ccstone's excellent post about similar.
Though I'd ask why you want to do this 
One of the reasons for putting things in the Dock is to make them easily mouse-able -- if you aren't going to use the mouse then you've other, possibly better, options available. For example, you could use a KM "Prompt for File" action or, with a bit more work, a "Prompt from List".