At the momment it's hardcoded. Note that you can only have 1 version of the page per server as skins are selected on a "per user" basis, but the user hasn't logged in yet, so DA won't know which skins to use for the login page.
Would the above Remote login page example work while incorporating a drop-down menu if you have several virtual boxes, or could we make the URL form post an inputted value also?