Is the point of this request to do verified activations? If so, then we're already working on a better version in TurboActivate 4.0.
If that's not what you're requesting, then I can't see the possible benefit to offering keys that are bought but then can't be activated. You could always implement this yourself using custom license fields. And revoking / removing a key that hasn't been activated by date X. But again, I'm not sure why you would want to implement this. I don't see how this could benefit you or your customers.