John,
About a year ago I created a custom implementation of what your are describing. I did it for an event registration pilot before Marketo actually had a way of keeping track of registrations with their iPad app.
My solution was the following:
Capture the registration on a Marketo form.
Send an auto email with the confirmation to the user (can also be sent as an alert internally if you want to create something to scan for IDs).
The email confirmation generated/included a QR Code image that would be unique for each registration. I did this by creating the image using Google's Chart API. I would use the API to generate the image/link using the attributes filled out for each user on the form.
That image can then be scanned with any QR Code reader (not only iPad QR code readers). The link could go to a page that you can create in Marketo that auto submits the entry into Marketo. The URL will contain URL parameters that would pre-fill the form with the values from the QR Code before submission.
I hope I am being clear and hope you can create that type of solution. It works like a charm.
Good luck!
Rafael