I haven't looked into any of the literature on sybil attacks, so I'm coming at this fairly blind, but would it makes sense to have individualized sybil fees based on our trust scores?

If we're zapping the same sort of content as other trusted stackers, then it's unlikely that we're an attacker.

Perhaps this could also be solved through a new post type: something like bounties, but with an unspecified payout and an ability for multiple accounts to pay in feelessly.

I can see the argument for not treating some of these zaps the same as ordinary V4V zaps, since they aren't sending the same sort of quality signal about the content, but it really would be nice to not have to do these transactions separately.