RE: Although subjectively downvoting posts to zero is anathema to me, the Layer 1 ability to do so must remain (for now at least)

Ah, yes. I had not seen that before. Thanks for sharing!

I was getting ready to publish a post explaining how the only way to effectively counter subjective downvotes is to fight DVs with DVs; and then recommending against doing so, because it would ultimately hurt much more than it helps, due to all the collateral damage associated with every DV (i.e. DVs reduce or eliminate curation rewards from those who cast upvotes in good faith). Not to mention the fact that creating a DV warzone would be a really bad look for the platform.

Your proposed solution enables a counter that should be effective (or at least partially effective, depending upon the relative stakes of all involved) but without the collateral damage.

One potential problem I see with that solution involves timing. A malicious downvoter can time their downvote to occur just prior to payout, thus eliminating the opportunity for 'free upvotes' to counter the downvote. So, maybe free upvotes should be allowed up to 24 hours after the normal voting window closes.