I was going to reply but knackered last night!
I filled in the details but never got any form of confirmation or even a reply from usrentacar who I emailed.
So as there was no queue at all at Alamo, I went to the desk and they had no knowledge of me as they still took all my details.
It doesn't mean it doesn't work but I saw no sign of what I'd filled in online being on their computer or referred to on the invoice slip.
As I said though, absolutely no queue whatsoever so wasn't really bothered. It might be all change when Virgin use Alamo
Andy