ahh.. missed the trunk part. I'd agree, no easy way to do this with that design. There are some enhancements coming around switchport authentication but I haven't seen anything immediate to address this.
I can think of a couple ideas to solve this using standards based approach. I also have some ideas using tags and Meraki Authentication. All would require development on our part.
We welcome the feedback on this issue. We can share your ideas back to the team on how this could be solved the easiest way possible?