Hi Blair and Don,
You don't have to set up a new role. You can exploit the existing 'authenticated user on the front page' role.
More details is available at: http://docs.moodle.org/22/en/Blocks_FAQ#How_do_I_make_a_front_page_block_visible_to_teachers_but_not_students.3F
With more than 11,000 users, you may want to export the list of teachers using Bulk user actions (Site administration > Users > Accounts > Bulk user actions > Show Advanced > System role > Teacher) etc.
Once you've got the csv file, you may want to do csv upload as Teresa suggested.
Another solution is:
1. Create a new custom role for the front page block, for example: 'Frontiers' or use the existing custom role in Don's case;
2. Create a dummy course where the existing teachers can self enroll;
3. Set the course with enrolment key, so only teachers with the right enrolment key can enrol.
4. Set the 'Default assigned role' in the self-enrolment module to be 'Frontiers';
5. Instruct the existing teachers to self enroll to the dummy course with the enrolment key.
By the way, the Bulk user actions' Message functionality doesn't seeem to work properly
Kind regards
Adi