I would think it would be a tough project. The heart of the plane...and the most critical part is indeed the sole of the plane and then the frog...in that order. If you have a Mutt plane and the frog is questionable, even adding a beautiful infilled wood would just mean you have a beautiful, but crappy performing hand plane.
Granted if you were real creative, you could also modify the sole and stiffen the frog with more iron through careful welding, some surface grinding and other machinists type tricks, but...if you are going to do that, then why not just start from scratch and make a plane?
In some ways Paul Hubbman and I did what you were saying with his 1909 Stanley #140. I was working at Lie Nielsen at the time and through machining and rebuilding made a lack-luster Stanley #140 into a fairly respectable handplane. It was never made for some of the modifications we made on it, so it was not perfect, but there is no doubt what you are saying can be done. In fact we did it with Paul's plane...but to be honest with you, a Lie Nielsen #140 right out of the box would perform as equally as well and be a lot less hassle to get.
Would I do it again? Maybe. It was a fun project and I have a clear conscientious as I know Paul's son is going to inherit a one of a kind handplane. A hand plane with history and a story, and you just cannot buy that.
You can read about the entire rebuild of that plane here:
I have no intention of traveling from birth to the grave in a manicured and well preserved body; but rather I will skid in sideways, totally beat up, completely worn out, utterly exhausted and jump off my tractor and loudly yell, "Wow, this is what it took to feed a nation!"