You said no one's figured them out? I have. Mwahahaha...
erm, anyways...
For the Kobe signature layup, this is where i learned how to do it from:
http://hoopsgaming.com/showthread.php?t=4917It works, I've tried it myself. the important thing to remember is that you have to BEAT your initial defender first i.e: have your initial defender behind you or beside you, not in front, and then like the guy in the link said, you have to be roughly in that exact area of the court. the button press is as he said (shot stick AWAY FROM THE BASELINE). after some practicing you'll get it down too.
But something that the guy didn't mention, is that even if you're able to perform it, you'd probably miss all your shots with it at first, because (and this is important): the shot release timing is LATER than what you might intuitively believe after seeing the animation. So basically hold the shot button a little longer (and definitely do not release early) for the shot to go in. Cost me a lot of time before i obtain this bit of knowledge... Go to practice mode to practice the timing.
Contrary to what the guy who made the post and most people believe, you don't have to do the 'post move' practice mode, apparently you don't have to have a defender for this signature layup to appear. Just 'freestyle practice' would be fine. Just remember the positioning and button press.
For Jamal Crawford's signature layup, this is where I learned it from:
http://www.youtube.com/watch?v=KrCC7q0NYwkExplanation:
-you have to be roughly in that area shown in the video for the layup to be triggered.
-like with the Kobe signature layup, you have to have your initial defender beat.
-the 'arrow' sign in the instruction refers to the RIGHT SIDE OF YOUR PLAYER, since the Jamal Crawford in the video's dribbling the ball with his right hand. So if you're using Broadcast cam or any other sideways camera, you actually press DOWN on the shot stick, which, if your player is dribbling with his right hand, would be his right side, as per the instruction.
- unlike Kobe's signature layup, I think this layup has to be practiced in 'post moves' practice mode, it seems you need to have defenders for this layup to trigger.
-I'm not sure, but i think the secondary defender has to be positioned like in the video as well, (which is on your left), but when you try to do this layup you will usually find yourself in that position naturally anyway.
I've also tried and practiced this layup myself, so i can assure you it works. (My guy in MyPlayer exclusively uses Jamal Crawford's layup package now, because now I can pretty much bust those layups out at will

)
Oh, and as a bonus:
Dr. J.'s signature layup is basically one of his reverse layups, so you do it by pressing the shotstick TOWARDS THE BASELINE when you're running to the basket on the baseline. The only thing you have to remember is that your dribbling hand must be the hand that's closer to the baseline. Oh, and don't start from too close to the basket. You can practice it in 'Freestyle' practice mode.
Steve Nash's signature layup is one of his normal layups (you press the shotstick TOWARDS the basket), it will trigger randomly. You can practice it in 'Freestyle' practice mode.
Hope that helps, and good luck!
PS: Now if someone can only show me how to do Rondo's signature layup in just as much detail, I'd be really grateful.