The stable-maint team manages the stable branch for OpenStack core projects with the support of the core developers of those projects and the release team. From https://wiki.openstack.org/wiki/StableBranch

the stable branch is intended to be a safe source of fixes for high impact bugs and security issues which have been fixed on master since a given release.

So releasing new features in the stable branch would not be a good thing. You can definitely provide your plugin to your customers outside of the main OpenStack trunk and keep it in sync with the stable branch. I would suggest you to work early in the release cycle with the Cinder PTL to include the driver in the next release.