Hi, Ex BBC guy here. We used to use something called “Edivue Diluent” to ‘read’ magnetic tape. It was a suspension of very fine iron filings in a volatile solvent, made, I think by Ampex for manual(!) VT editing. We also had ‘pelicles’. Again, iron filings suspended in solvant behind a very thin membrane. Both allowed you to ‘develop’ the magnetic image on the tape. But, to be honest, for your purposes, you’d be better off tweaking azimuth by ear.