Do we have a bug in Gerrit yet to support this kind of extension point?

No. Now that i'm gerrit member i can file one.

WIP patches:
  https://chromium-review.googlesource.com/c/412746/ for the plugin
  https://gerrit-review.googlesource.com/c/91991/ in Gerrit

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/gerrit-plugins/git-numberer/+/59efbefa2d2844d3fd31cc83a6d27d6b36facf3d

commit 59efbefa2d2844d3fd31cc83a6d27d6b36facf3d
Author: Andrii Shyshkalov <tandrii@chromium.org>
Date: Wed Nov 30 17:33:39 2016

Add Generator plugin config.

Generalize existing Validator.isEnabled check and refactor test.

R=ekempin@google.com
BUG= chromium:642484 

Change-Id: I733c5969eef08d6e8a85f9181a149f0f8ec5d815
Reviewed-on: https://chromium-review.googlesource.com/414909
Reviewed-by: Edwin Kempin <ekempin@google.com>

[modify] https://crrev.com/59efbefa2d2844d3fd31cc83a6d27d6b36facf3d/src/main/java/com/googlesource/chromium/plugins/gitnumberer/GitNumbererModule.java
[modify] https://crrev.com/59efbefa2d2844d3fd31cc83a6d27d6b36facf3d/src/test/java/com/googlesource/chromium/plugins/gitnumberer/ConfigTest.java
[modify] https://crrev.com/59efbefa2d2844d3fd31cc83a6d27d6b36facf3d/src/main/resources/Documentation/config.md
[rename] https://crrev.com/59efbefa2d2844d3fd31cc83a6d27d6b36facf3d/src/main/java/com/googlesource/chromium/plugins/gitnumberer/RefGlobMatcher.java
[modify] https://crrev.com/59efbefa2d2844d3fd31cc83a6d27d6b36facf3d/src/main/java/com/googlesource/chromium/plugins/gitnumberer/Validator.java
[modify] https://crrev.com/59efbefa2d2844d3fd31cc83a6d27d6b36facf3d/src/main/java/com/googlesource/chromium/plugins/gitnumberer/Generator.java

Update:

while generation works, the code is called *after* currently hooked validation code, which means that if validator is enabled, the generation code is never called.
hence, the solution is to create new validation hook which is called precisely after new commit is generated but before the destination branch ref is updated to point to it.

Very WIP https://gerrit-review.googlesource.com/c/93650/ with detailed reasoning. It has to be reworked (see Dave comments on it).

new extension point of Gerrit landed: https://gerrit-review.googlesource.com/c/94436/

4 Cls to make use of it are being landed (2/4 landed at this time)
https://chromium-review.googlesource.com/c/430826/5
https://chromium-review.googlesource.com/c/430904/3
https://chromium-review.googlesource.com/c/430932/3
https://chromium-review.googlesource.com/c/430909/1


What's next?
land them + deploy. And then enable Generator on refs/heads/master and refs/branch-heads/* in chromium repo :)

I think everything has landed as I intended. Waiting for this to be deployed.

It has been deployed. Tested and it works. Tried branching including that those outside of refs/heads/. Example: https://chromium.googlesource.com/playground/gerrit-cq/normal/+/refs/gitnum-heads/branch2

Enabled generator for v8/v8: https://chromium.googlesource.com/v8/v8/+/81d10877c20705062483930d0ac09f7b0fd27c6a%5E%21

Enabled generator for chromium/src: 
https://chromium.googlesource.com/chromium/src/+/03cdeaa4994ae3bee6c53df237d5238608596f51%5E%21/

Enabled generator for external/webrtc: 
https://chromium.googlesource.com/external/webrtc/+/e1514e3d17e4875f498765e8ce8b6a1fcbcf7e0e%5E%21/#F0

From Git-Numberer PoV, these repos are ready for dogfood.