Currently, the codex repo in gerrit is set up to inherit from the design project, and have the owner be the design-codex group. That group includes @AnneT @Catrope @egardner @Tonina_Zhelyazkova_WMDE @Volker_E. From the design project a few other people get merge rights.
On the other hand, WVUI had the mediawiki group included in its project owners group, allowing people with +2 on general mediawiki/* repos to also +2 changes to WVUI. I propose doing the same for design-codex, i.e. add the mediawiki group as a member of the design-codex group.