This is a very interesting idea. I think the biggest objection to it is the added processing cost for witnesses to correctly calculate upvote rewards when writing votes into the blockchain. Maintaining an upvote map of the last 100 votes for 500k accounts...
This is a nontrivial processing load...
Yeah, unfortunately I don't have enough experience with coding to be able to think of a better way. I'm not sure if it would be easier to keep an upvote map for each person or to look it up and calculate it at each vote... I'll be looking more into how this can be done