Is it no longer possible to send someone a Motion project to drop on a timeline?
No, I hope they get around to enabling that sort of integration. Sounds ideal.
Definitely can't do it as of FCPX 10.0.7 and Motion 5.0.6
They must now integrate it into the FCPX Generators?
No, I think that was a real "workaround" solution. I wouldn't be publishing generators for every video I created in Motion. It's for creating templates, not a subsitute for project integration.
The only way is to export (usually ProRes 4444 if you need the transparency)