I would be game for this if we can get mastery tokens above level 5 in ARAM as well.
I got S+ ratings in ARAM on several champions, often by sheer happenstance, but as some of them are classified as junglers, I wouldn't dream of taking them to that other map. Champions like Elise, Olaf and Trundle.
Perhaps give mastery tokens above level 5 when getting S- ranks or above a set number of times, with the number being different depending on the map.
Yes, it still would be down to chance at times, but when getting S+ rating and with a 3 K gold lead over the next player, and still only get some 170 mastery points instead of 700 if we lose, the value of actual skill in getting S+ ratings feels rather contradictive.