It can be done, but it's painful
I eventually registered my N7 after about the 10th attempt. You need the 12-digit SSN serial number beginning with a "C" from the N7 cardboard box (the 16-digit hex serial number displayed in the Settings and on the cardbioard box as "SSSN" - which the Asus site stupidly actually tells you use - is *not* the serial number they're after). Folks on the HUKD forum cleverly suggested installing a barcode reader app and using that to scan the barcode below the SSN on the cardboard box.
The latest snag is that I never got the e-mail instructions for sending the proof of purchase, but apparently you send it to firstname.lastname@example.org - or, actually, you don't until at least Monday, because the buffoons at Asus now have a full inbox and all e-mails to that address are bouncing!
Looking at the Asus store, you get 19 quid to spend (postage is a minimum of 6 quid), so that rules out getting the charger/cable for free, so I guess it's one of the cases then. The store is so expensive (typically twice the price of anywhere else), that there's very little you can get for 19 quid that's worthwhile and the aggro I've had with this has probably exceeded 19 quid's worth of my time!