I'm neutral about applying it to 2.7, though I can see the value in doing so.

For 3.5 it definitely has to modify _msvccompiler - the other two modules are just left behind to distract people apparently :) (but actually in case someone accidentally(?) too a direct dependency on an undocumented class).

One thing I'd like to see is an environment variable to switch it on - probably DISTUTILS_USE_SDK is fine for this. In the default case, we know exactly where we are looking for the tools, and so we know what they are called. If you're intending to override this completely, you probably need to override everything, and DISTUTILS_USE_SDK is the flag for this.