Login with only registration number

Hi,

I would love to be able to give users the ability to login with only a registration number.
For example. Flyer by my hometown doctor, visit my site and do a survey, and then have on each flyer a unique number. If they login to the site with this number, they got a survey to fill, and I can keep statisctics only by the flyer number, so privacy is guaranteed.

Thanks a lo in advance for your reply!

Greetings,
Martijn