I was under the impression that the Choate stock is a solid tube, thus making it difficult to make a hole and still retain structural integrity, a few pics may help clarify this to me.
I got the ATI Plastic Top folder and ATI plastic Side folder that I interchange ocasionally.
The ATI Top folder can be milled if needed and the Side folder won't interfere at all.
I had a Choate Stock for my AK and it was top quality and very rugged, but the ATI stocks are really rugged and weather proof too and lighter than the Choate.
The moral of all this is that IMHO, the sight system shouldn't be modified, but the stock may be modified or replaced if needed for your application.
Here are pics of my top folder