I think that's what I've done. I changed every single modelb column in each row that could possibly (to my untrained perception, anyway) be the male player character(s) but my changes have not had any effect.
My guess (again, an uneducated one, because I am super new to the whole KOTOR modding scene as a whole) is that it has something to do with the fact that I'm playing the Steam version which I understand has several hiccups when it comes to modding, or how I'm using a modded player head or something. Again, I don't really know.
Does the choice of player portrait affect which row in appearance.2da needs to be changed? If so, is there any way for me to cross-reference and ascertain which row I need, corresponding with my character's selected head?
As for there being a mod that already exists, if there is one then it has eluded my search for many, many years. I've wanted this to be a thing since I first played TSL in the mid 2000s and I've never been able to find a way to do it.