This idea has been developed and deployed to Canvas
I just spoke with a Canvas support person and they verified that there is no way to have extra credit items in a Rubric such that the overall score will be greater than the defined perfect score for the assignment.
For example lets say I have an assignment worth 100 points. I then have two extra credit items a student can implement, the first worth a maximum of 5 points the second worth a maximum of 10 points. I would like to add two Rubric items for each extra credit and allow the TA to put the points associated with the extra credit results in each of those items. This should result in a student getting up to a 115 out of 100 on an assignment that has a perfect score of 100. This does not happen currently. When the TA attempts to do this the score remains at 100. The TA then has to put a comment in indicating which extra credit they received points for and then adjust the overall score. This is error prone and not ideal.