On the implementation side a bot (likely this new one: https://github.com/tvm-bot) with triage-level permissions could ensure that [skip ci] is present in the PR title if any of the commits in the PR have skipped CI (if a commit comes later on where CI isn’t skipped, the submitter can manually remove the tag from the PR and the bot can remove the label), similar to other Apache repos with triaging bots on PRs. Optionally it could add a label as well (which has the benefit that submitters without committer status can’t edit labels directly)
It seems like it would be simpler for us to rely on both the PR title and the commit being built starting with [skip ci] (with a clear indication of why CI was skipped or not skipped in the CI logs). This would obviate the need for a bot to be writing PR titles or adding labels. We still may need the bot to authenticate though in lieu of installing a GitHub App to the repo.