I share your doubts - modifying something within the OSM magnifying glass would hardly be recognizable in the favicon size and probably make a physical magnifiying glass necessary. If you are looking for clearly distinguishable favicons for your browser tabs you would need something entirely different in my opinion.

If you want to stick to the OSM magnifying glass probably different colour backgrounds could be a solution?

Anyhow for me this would not make too much sense, I am quite happy with the same favicon for all different OSM-related browser tabs.