Please let me jump into your thread since I have the same problem.
I have a Mori Seiki SL-3 with fanuc 5T.
In my machine after I hit the ON button on the back cabinet, it used to turn the front panel on and there is this led under the turret index selector that is supposed to be on; after that I just had to hit CONTROL ON and I was ready to go.
Now I have the same problem you have, I can not turn CONTROL ON.
There is this signal called PRDY2 and is located on the main board, top left, rigth besides PRDY1, both of them have to have 24 VDC otherwise, well, our problem. At this time I only have PRDY1 with 24 VDC.
This signal comes from the servo drives and I think both of them have to produce their PRDY2 signal (but that is just a guess). I do not know if the drives have to become hot after you hit the back ON button, but in my case is not happening. You mention your drives' MCCs turn on and off, maybe you have a problem with the voltage being suplied to the MCCs.
Anyway if you or any reader find the solution pleas let us know.
I hope this help you a bit, maybe find the solution and help me back. If I fix it I'll let you know.