As I mentioned in my previous post, there is an apparent bug in the Metaweblog API that prevents me from specifying the full path to the FxCopCmd executable. I'm not entirely sure what is actually happening, but I do know that it doesn't allow the post to be submitted through either Windows Live Writer or through the blog Admin UI.
The strangest part of this is that it appears to only care about this particular executable. I've tried several others, both command line and GUI applications and they all work just fine. Even stranger is that I can specify the full path to the GUI version of FxCop without any problems.