Good post! Thanks for chiming in CT.
Of course people have brought up muscle groups like this. But from what I've seen, nearly all those who post these sorts of inquires on message boards are nowhere near the development of someone who needs such specialization for simple appearance or competition. And in many cases, the poster has such weaknesses simply because their programming is off and the muscle is not being trained with the right exercise for their structure, not being given priority by placing it in the middle or end of a workout, or with too little volume or frequency. Granted, early on, a discernible person can within months or the first year of training, see which muscles are going to be stubborn and which are going to grow and become stronger with ease, but I think someone should really put in some time before specialization.
You have far more knowledge and experience and development than I do, and ever will, but the way I see it is this. The formula for genetically ordinary people goes something like this, and I've posted it so many times. One can take notice that all the popular upper-lower splits like PHAT, Candito's Hypertrophy, PHUL, even most of the 5/3/1 program, and Lyle McDonald's GBR programs all follow this to a T or close to it.
Frequency: 2 x per week for each muscle, or a split that allows every 4 to 5 days
Number of exercises: 1 or 2 for each muscle, use nearly all compounds, except for arms, calves, and stubborn larger muscle groups
Sets: 2 to 4
Reps: mostly 6 to 8, 8 to 10, higher for arms and legs sometimes (total of 25 to 50 reps per muscle per workout)
Split: upper-lower, push-pull-legs, or whatever allows you to use the proper frequency
Add weight and reps over time and there is no way an ORDINARY frustrated bodybuilder is not going to grow, rather than mess around with advanced specialization and programming. I say this, because I am ordinary (maybe a tad bit better genetics than ordinary... maybe) and most of my life have been around mostly genetically ordinary men.