What your describing IS grain filling. There's no other way to do it. Mahogany absorbs dyes too well and the stain and sand back method doesn't work.
I've been building guitars a long time. How I said it is how it's done. You're obviously welcome to explore other avenues, and I won't stop you from wasting your time.
So is it safe to put the whole body of the plane in evaporust? Didn't know if that would mess up the remaining japaning or anything, it's pretty rusty. Hoping since I can't do any woodworking for awhile since my shop is in storage, trying to find something to do.