You're right to not want to do this via direct SQL, as you're ignoring a lot of other data validation steps that will cause errors in your data.

You could do this out of the box in OpenERP v7.  In Odoo v8 this isn't possible without custom code due to how quants work.  You'd need to override the existing action_cancel() function in the Python code to bypass the error for the done status, then undo changes to the quant the item came from.

What might be smarter and easier is to add a function to quickly create a new stock move that reverses the transaction.  Basically, it needs to duplicate the original stock move, then swap the source and destination locations, and immediately finalize the move.  This will also resolve the quant accuracy issue, and ensure that any accounting valuation entries are reversed as well.