Oh, shoot. I haven't been looking at Surrender@20 lately and just did, and see that they have a Red post concerning Mastery in the Match History.
http://www.surrenderat20.net/2015/07/red-post-collection-august-sale.html#3
In a reddit thread asking for Champion Mastery grades to display on match history, Socrates replied:
"We want to make this happen at some point. Can't give you guys a date yet but it's something we're looking at as part of the next iteration for champ mastery."
As for who might be able to see can see these potential grades on match history, he noted:
"We aren't likely to make it completely open since those games where you land a D+ aren't really worthy of sharing.
Being able to call out when you crush it and land those S tier grades would be sweet though."
He also briefly commented on the likelihood of seeing the grading /mastery system added to ARAM:
"The complexity with this is the grading system for ARAM would have to be pretty different since the game mode is very different. We're not building that atm, but that's not to say we won't ever do it."
In response to a "Soom tm" to his comment about making this happen at some point, Socrates explained balancing the addition of features and teased the team is currently working on a larger champion mastery feature:
**"This is a sensitive topic, but I'll try to give you guys a satisfactory response.
When we see opportunities like this we can focus on it to deliver some short term value. The trade there is some longer term value gets bumped off or interrupted. For example, right now this team is focused on a larger feature related to champion mastery that we think you guys will be excited about, but something that takes longer to build. **
**It's kind of like a strategy game. We could zerg rush and deliver a small feature. The cost is late game units. **
Building games is really all about making these trades. We don't always do it perfectly and you guys keep us honest."
