In latest UMPIO gig I saw 2 weeks ago in Turku/Finland, there was this amusing moment, where he noticed that one of the source sound tapes didn't work. So he just took the tape from deck, pulled out loooong piece of tape, and started rewind in back into tape with his finger.
I do a fair amount of cassette abuse with four tracks, and anytime I'm recording or jamming and the tape fucks up and starts getting eaten etc. I make a point of continuing recording while manually fixing the tape as practice so when it happens live it not gonna phase me or have the noise eat shit.
Re the OP, I've experimented with magnets for tape degradation and the results have been underwhelming, that's all I got.