Nimbostratus
CirrostratusIs this the first page in the VPE? How are you populating that variable before they've put in their user/pass to do the AD lookup? You could add a popup box before that page to make sure the variable is set.
The pages are all php that gets processed before it's sent out, so as long as the variable is formatted properly (sounds like it is), it should automatically show up as a select input. If you look through the rest of the PHP I'm sure there's a function that expands each of them depending on type, etc.
Try adding a second login page after you've done the first login and AD query, and on that second page put the dropdown with the custom variable.