If it's got the same Hadley air suspension as my KGDB, then yes, you can override it. Just push and hold the down arrow on the control panel. You'll probably get at least 2" that way. Just go dead slow when you do. The suspension automatically raises over a certain speed.
I would also recommend a spotter the first couple times you do this, just to make sure you're clearing the door.
If it was me, I'd probably install a wireless webcam pointed at the door opening, with a tablet or laptop on the dash so I could see the camera feed as I went under. But I'm a geek.