I've been thinking about this idea of yours for a while now and I have a counter proposal. Why not have commenting or even posting power that gets depleted much like our voting power does right now. Or course, no slider and when it falls to a certain percentage, you can't post anymore until it regenerates. It can probably be adjusted in such a way that you can still comment relatively actively, but still would put the brake on accounts that create an excessive amount of comments and this is what the spammers need for their operation to be successful.
And if you have depleted your commenting/posting power, you can burn some steem to regenerate if you don't want to wait. And I do mean burn, not pay a fee to somebody. So it can counter a tiny bit of inflation too.
I was thinking that getting upvotes on your comments might also increase your commenting/posting power by a bit, but I'm not 100% sure it's a good idea. Maybe it is.
There is already a similar concept, called bandwidth in the Steemit whitepaper. Did you read it?
I read it months ago, but a lot of it flew over my head back then. But I have heard of the bandwidth thing, but as far as I remember, it was tied to the Steem Power you have. But yeah, maybe modification of the bandwidth thing that is already part of the system might be a solution I would be more likely to support.