I suppose there is some room for experimentation with coating for white cotton tape. The nice thing about shellac is that it remains fairly grippy for a glossy coating, and it's organic I guess.

That said there are plenty of absolutely clear coatings that could also be tried. Perhaps a silicone type waterproofing spray would help a bit. Acrylic and polyester clears can be absolutely transparent. Would they make the bars slippery? I don't know.

As an aside that may be of some interest: I clearly remember when I first saw a Singer in real life, about 1989 IIRC. The first thing that stood out was the strange shiny cotton tape. At the time it was totally exotic and crazy!!! My fellow LBS workers had a vigorous debate about what the coating was. It took a couple months to figure it out. This was in spite of the unhelpful answer of 'vernis' directly from Ernest at Singer...