isn't champion mastery, based on the champion your playing? if 1/1/18 was in the upper levels for champ X at y time. that would more likely score an S rank, compared to someone normally played in a different role.
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It means Riot won't go for it.
If they added a ward check per minute, stacks of support item used, and builds. That might work a bit better than what you have proposed.
That's really wouldn't achieve the effect Riot is looking for. Partially since Wards are not your champion. Using such a system would likely give an S rank to anyone who uses Wards. Also, it wouldn't measure the quality of ward placement, it would simply be a matter of machine gunning wards.
Everything said, I think the best thing to do for supports, is to make sure every reasonable exploit, (whether it was a champion ability or an item) granted the Support an assist.