I've noticed that @jow merged the backend uci configuration for VLANs with DSA switches in August https://github.com/openwrt/openwrt/commit/96b87196b0788d4cdaa81a49a65d198d9f6c90d2
more information about it and examples are in the cover letter of the RFC patches on the mailing list (which may or may not be still correct after the feedback in the discussion that followed) https://firstname.lastname@example.org/msg53505.html
Has anyone tried this? I don't have much time for tinkering at the moment.
Would be nice to have some basic instructions and examples to put in the wiki about this.