Geoffrey suggested using the existing label "do not merge yet" which I've been applying manually to provisional PRs:
https://github.com/w3c/web-platform-tests/pulls?utf8=%E2%9C%93&q=is%3Apr%20label%3Achromium-export%20is%3Aopen%20label%3A%22do%20not%20merge%20yet%22

Here's a CL to automatically add it:
https://chromium-review.googlesource.com/c/517400/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3daa00a0d514c5b9c6d69aa3a669843ee3d9aaae

commit 3daa00a0d514c5b9c6d69aa3a669843ee3d9aaae
Author: Jeff Carpenter <jeffcarp@chromium.org>
Date: Wed May 31 15:48:05 2017

[WPT Export] Add "do not merge yet" label after creating Gerrit PR

Bug:  721847 
Change-Id: If7cff35d4f827ee1956b3378d3246ea12b7f749e
Reviewed-on: https://chromium-review.googlesource.com/517400
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Jeff Carpenter <jeffcarp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#475922}
[modify] https://crrev.com/3daa00a0d514c5b9c6d69aa3a669843ee3d9aaae/third_party/WebKit/Tools/Scripts/webkitpy/w3c/test_exporter.py
[modify] https://crrev.com/3daa00a0d514c5b9c6d69aa3a669843ee3d9aaae/third_party/WebKit/Tools/Scripts/webkitpy/w3c/test_exporter_unittest.py
[modify] https://crrev.com/3daa00a0d514c5b9c6d69aa3a669843ee3d9aaae/third_party/WebKit/Tools/Scripts/webkitpy/w3c/wpt_github.py
[modify] https://crrev.com/3daa00a0d514c5b9c6d69aa3a669843ee3d9aaae/third_party/WebKit/Tools/Scripts/webkitpy/w3c/wpt_github_mock.py

This is half solved as of crrev.com/c/517400 above. There is still no step to remove the "do not merge yet" label once a CL has landed, which I've been doing manually. I'll keep this bug open for that task.

Removing the provisional label as mentioned in #3 is now done.