One thing I've desperately wanted from git worktrees is to use reflinking in some way to lower the cost of copies.

I just found https://github.com/josharian/git-cow-worktree that doesn't inspire confidence.

You could try proposing a worktree copy command on the mailing list. Worktrees are currently independent and work off of repositories which may have no existing worktree / working copy at all, let alone one it makes sense to start from, so there is bothing to copy but for dodgy heuristics.