I think the problem is that, with a high-mounted compass, you are
looking out the window every time you look at the compass. I'm not
sure how examiners deal with this.

From the Instrument Rating PTS:

Applicants may have an unfair advantage during performance of the TASK using
the backup flight instruments during an instrument approach due to the
location of the magnetic compass in some aircraft. When crosschecking the
magnetic compass heading, a view of the runway or other visual clue may be
sighted. It is the examiner's responsibility to determine if the applicant is
receiving visual clues from outside the cockpit. If an examiner feels that the
applicant is receiving visual clues, the examiner may devise other options to
limit the applicant's view. By no means shall the examiner limit his or her
view as the safety pilot.