Hi Andrew,
I have good news - this feature is currently in development! We're adding audiences as a manual rating method for competencies, to be included in version 20. This means you'll be able to add the group os senior employees to an audience, that is then assigned as raters for each of your specified competencies.
They'll have the same rating experience as managers (use the 'rate others competencies' link found on the user profile page, or set it as a primary navigation link), meaning they'll choose the person to rate and then the relevant competency.
If you have any questions about this please reach out, I'm happy to help!
Cheers,
Ashley (Product Manager)