If Apple allowed vendors to import any user as a "legacy user", then that's a clear and supported way for vendors to get around Apple's 30% cut.
http://news.ycombinator.com/item?id=4037065
As for how you do it, in practice 99% of licensing schemes are based either on "serial number", "mac address", "license key given to user and typed into app", or a combination of such. While some people then can't import still, almost everyone can, and that is enough to be game-changing.
Note: you don't even need to allow this list to e changed after you import the product if you are still concerned about a "30% cut". (Though, Cydia does allow this, and even fully automates it by way of a backend licensing API vendors can implement to take control of the purchasing process, and in practice vendors that start using this mostly use it as a temporary measure and eventually just switch over entirely).
That is not to say I wouldn't want this. I think it would be wonderful, but I wouldn't bet on it.
Apple, however, doesn't want to be on the hook for maintaining and supporting and spending bandwidth on users who haven't given them a cent. I can't blame them. Again, I'd love to be granted the ability to bless "legacy users" into the Mac App Store, but I also don't understand why Apple has any incentive to ever allow me to do so. It costs them money and resources, with absolutely no benefit in return.
Why do you think they have an incentive?
I think this is a overreaction to unregulated markets where merchants are constantly playing pricing games. As a consumer, it feels unfair when different people pay different prices depending on what hoops they've jumped through. However, Apple's approach feels unfair to me too, particularly the lack of trials and paid upgrades. I feel that this pushes everything towards "disposable" software rather than products that could grow over many revisions supported by upgrade from their users.