It seems to me that the ‘Used By …’ function would be even more useful if it also reported if the glyph was used by an Open Type feature.
I don’t know if this would be easy or hard to implement but it would enhance the program.
You can already use the more advanced OpenType Explorer to see in what OpenType features a glyph is used.
You can click a specific item in the tree view to jump right at it in the OpenType Designer.