In the old days, when draftsmen worked with pencil on paper, they used rubber erasers to remove pencil lines. To ensure that they didn't remove pencil lines adjacent to those they wanted to erase, they used "eraser shields". They sold for two or three bucks and consisted of thin metal with a slot in the middle.
Fast forward to the 2000's and StewMac started selling eraser shields for use on guitar frets - they didn't call them that. The slot in the metal shield is fit over/around the fret - the fret pokes through the slot - while the wood adjacent to the fret is protected by the metal shield.
You've invented the eraser shield. Potentially a good idea, just not a new one. I've never had a need for one in guitar work.
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