Graphite of course the project has had contributors outside the core team.
As an example of a project that has been considered and continues to be of interest, check out Florisboard. This is a permissively licensed project, which has had contributions the other way from GrapheneOS devs with the view to integrate it as an AOSP Keyboard replacement when complete.
Lead Devs position:
[...] given up on anything but permissive licenses and will primarily support and contribute to projects with permissive licenses.
Bad actors violate the licenses anyway and it gets in the way of using it via license incompatibilities and other painful restrictions.
Any permissively licensed project can't take GPL code because it would change their overall license to a restrictive one.
GrapheneOS has a strict policy against integrating GPLv3 code because it prevents building things we want people to be able to build with our software.