Refund contribution

I'd like to add a "refund contribution" action to Civi core, which does the following:
a) Create a negative contribution which is linked to the original contribution and contact. I'm not sure how to link the contributions, since transaction_id must be unique.
b) Fire a hook which can alter any of the records involved in the refund.
c) Perform the actual refund through the payment processor if possible, otherwise the negative contribution is left in the "pending" state.

Ideally, this action can be triggered from the "Edit Contribution" page.