I've used it once, which was on a night ride. My experience: Mr. Legg phones ahead to give them our ETA and when we arrive at the crossing there's no one there, so Mr. Legg uses the phone at the crossing to inform them of our presence. We then stand around for 10 minutes and a vehicle with a bike rack appears. We then dump the bikes on the rack and I try not to fall asleep on the 5 minutes crossing.
For a few of your it's fine and it doesn't take very long. I am informed it works less well when there's a group of 60 cyclists at 3 am.
I've no idea how it'd work in regardless to an Audax DIY.
Edited for spelling