If a player is used regularly the tracker bar will stay nicely polished
on it own.  Just the ends where the rolls does not ride may need
touching up with a polishing cloth before they get heavily oxidized.

It is important that the tracker bar be very smooth and not cause
the roll paper to drag.  I would recommend only the thinnest of
coatings carefully applied and test first to be sure they provide
no tackiness.  If applied on a test surface and allowed to dry the
surface should feel as friction free as polished brass or you may have
problems.

My preference would be a wax specifically formulated for metals
including brass.  I have a 10 pound can of something but forget the
name of it and it is hundreds of miles away so I can not check but
there are several waxes made for brass.

There are also clear finishes specifically made for copper bearing
alloys.  One name that I remember is Incralac.  It is used on museum
quality sculptures and is supposed to be harmless and stable.  I would
still check to make sure it dries to a friction free surface.

If you are just concerned with appearance, you could mask the area of
the tracker bar that the paper passes over and not seal this as it will
stay polished for many years if rolls are played regularly.

Best regards,
Spencer Chase