One barrier for this general approach: the basic argument that something like this would work is that if one shard is aligned, and every shard has veto power over changes (similar to the setup in Why Subagents?), then things can't get much worse for humanity. We may fall well short of our universe-scale potential, but at least X-risk is out.

Problem is, that argument requires basically-perfect alignment of the one shard (or possibly a set of shards which together basically-perfectly represent human values). If we try to weaken it to e.g. a bunch of shards which each imperfectly capture different aspects of human values, with different imperfections, then there's possibly changes which Goodhart all of the shards simultaneously. Indeed, I'd expect that to be a pretty strong default outcome.