Have you tried blending between two normals using LERP? And then setting the muscular version normals blend value based on muscle morph size?
I assume that was simple enough to understand my meaning, you can blend normals easily enough. Just make sure you use LERP and not multiply. And set the A of the LERP to blue (which would be nothing) and the alpha to your scaling parameter.
I think I’m remembering how to do that correctly. It’s been a while since I’ve messed around with materials. But that’s what I can think of off the top of my head. I don’t know exactly what you’re looking for, but hopefully that’ll get you on the right track at least.