That looks very like the problem we've got. The SQL upgrade is tied into the same 'job' as the 1.6 upgrade so fingers crossed it'll get fixed - I'll post the outcome to this thread.
In the meantime, and in the spirit of maximising the use of this thread to any future confused person stumbling across it: I have been adding records to the mdl_user_teachers table by hand - taking the userid value from mdl_user's id field (identifying the Moodle user to be given teacher rights) and the course value from mdl_course's id field (identifying the course page). Save changes, return to your Moodle site and bingo! Be sure to let SQL generate the id field for mdl_user_teachers though, as it should be a unique identifier for that relationship.
Seems to be problem-free so far - don't believe any other data or function is dependent on this table, so outside-of-Moodle changes shouldn't be a problem - only doing by hand what Moodle would be able to do if SQL was behaving.