The first and easiest route is if you happen to know who had the iPad and maybe they'd be willing to type their password to unlock the device.
Otherwise, I think you're stuck with contacting Apple. If you've got enterprise apple support, they provide a generic spreadsheet that you fill out and email to a contact, usually turn it around within 24 hours. We utilized this when we sold off older iPad 2s that had to be unlocked for the purchaser.
If the iPads are in your DEP, that's proof that you own it.
