From the I can't believe I had to write that file comes this question from a customer:
Our customer is modifying the ABC.DLL file that comes with Windows in order to accomplish XYZ. Is this supported?
No, of course this isn't supported. I can't believe I had to write that. if you modify a system file, then the thing you're running isn't Windows any more but is rather some sort of operating system that resembles Windows in many significant ways.
(Imagine the extreme case of this: The customer modifies NTOSKRNL.EXE, KERNEL32.DLL, USER32.DLL, etc. so that they happen to be byte-for-byte identical to the files that shipped with Windows 2000. Does this mean that Microsoft supports Windows 2000?)
The customer is just looking for an official Microsoft statement that this is not supported. They know that it's a bad idea, but their client wanted to have this feature, so when the customer tells the client "No, we can't do this," they need some sort of justification.
Oh, I see now. The customer is chicken and wants Microsoft to be the bad cop when they deliver unfavorable news to their client.
My fear is that the company, by being spineless, is now subject to manipulation from their client. "What, you're not going to let big old Microsoft tell you what you can and cannot do, are you? Be a man! Implement the feature!" And then the company will buckle under the pressure and implement the feature anyway.
And then when the feature stops working at the next security hotfix (or worse, when security hotfixes stop working because you modified the file), the client will complain back to the customer, who will defensively say, "Um, yeah, stupid Microsoft broke the XYZ feature of our product. Bad! Bad Microsoft!" Or maybe the customer's plan is to change their name and move to another country so the client can't find them when everything falls apart.