I don't plan on any TMCC in the near future. The best thing is to go to the manufacturer an ask away. A secondary company could leave you hanging. Denny had the problem with sound boards he couldn't repair. The best thing about O is that you have the room for a decoder and the Williams has a DC motor.
I did a Williams E7. I put an electrocoupler on the front as well...I think I had to shim it a bit somehow. I didn't cut or drill any holes. The sound is just ok, comes out ok. I used ERR. Lots of room. Easy.
I upgraded a 2055 & 2056 steamer (Electronics go in tender, connected to the engine with a tether) and a 624 diesel (lots of room after e-unit removed). Its not a task for the faint of heart or someone not familiar with wiring and a soldering iron. ERR does offer a labor rate to do the modification for you....
On the 2056 & 624, other then removing the e-unit and whistle in the tender... I didn't have to make any modifications to the engine at all.... your results may vary, and asking ERR with your specific engine will let you know if you will run into any problems.
I have to say that I enjoy my layout much more now that I have control over individual components. I can run some interesting scenarios all from one control.