Given that contracts can be formed in the blockchain using scripts, couldn't a contract include a mechanism for facilitating chargebacks?
You could use 2-of-3 outputs to do this, where the third key is held by a trusted mediator.

A merchant could consider himself paid when the 2-of-3 transaction is confirmed. In the normal course of events the 'chargeback period' would expire and the mediator and merchant would send the money to the merchant's normal address.

If the customer requests a chargeback before that, then either the merchant would agree or the mediator would step in and perform the chargeback according to whatever policies it sets.