Performance Management - EE Rebuttal

We are currently in the midst of rolling out Performance Management for our professional areas.  Our second phase of implementation are our union employees.  During conversations with the union, they were asking about the opportunity for employees to sign the review, however to provide a rebuttal and having that rebuttal housed with the review. 

With our original roll-out, our idea was to have employees log into CRM and create a case with HR as a tracking of the rebuttal; however that doesn't help having the rebuttal and performance review in the same system. 

Just wanted to see if there are other companies have had to deal with rebuttals and how they have handled it.