Just add +[bar] to any gmail address, as in 'foo+[bar]@gmail.com'. Your registration system will treat it as unique, but gmail will route it to one email inbox for foo@gmail.com
Looks like Gmail doesn't like the [bar] solution though
I use it a lot for signing up for mailing lists actually so you can tell if a company has shared your email. If my address was example@example.com, I might sign up with example+zappos@example.com. It's always worked for me.
You've totally saved me tons of time and effort