There is a method to bypass the registration, but it's somewhat of a hassle as you'll need to repeat the steps for each firmware upgrade.
Instead I suggested you simply create a new email address at Gmail or Yahoo that you only use for times like this if you don't want to use one you already have. I have one of my numerous emails associated with my Kobo account and no other personal info at all, I never buy any books through my reader. I prefer to buy all my books from my PC only, then I sideload everything to my readers from Calibre (thanks to Apprentice Alf).
I always worried about losing the reader and whoever finds it able to purchase as many books as they'd like through my account. Without any billing or personal info tied to the account, that's not a problem. Most every ereader and reading app requires at least an email input, it's really not much of an imposition or anything out of the ordinary.
Edit:
Here's one thread about bypassing Kobo's registration in case you decide you want to do that instead. But as I said, you need to repeat it every firmware upgrade, and there's really nothing out of the ordinary in supplying an email to a site. You don't need to supply any real info for the email account you set up just for this purpose.