Hey Yoda
I have tried it before where I trained a capped player. I had a C that was capped. He was 22 years old and I was doing 2 pos training. He did not pop at all that season. I ended up giving up, selling him and used the cash to buy another trainee.
So simple conclusion: it is not worth training a player once you know he is capped.
Also, remember that the cap is based on how many skill points the player has and not salary, so the salary estimate scale is very off if you have a more rounded player.