I haven’t found any information about this. Do I understand correctly that Marmoset considers #7f7fff to be flat? Is there some way to adjust the standard flat color?
Flat normals in Toolbag (and most other applications that use normal maps) are 127.5, 127.5, 255. This is technically in between 7f7fff and 8080ff, but either of those should work. There is no way to remap the normal map input. Could you provide some more information about the problem you're trying to solve?