I can understand why you want to override the surface pattern, but what's so important to use a model pattern? Think about it....model patterns are supposed to represent actual model geometry but in "wireframe" format so to speak. So if you override a model pattern, in essence you're changing the model. I believe that's why you can only use a drafting pattern. This way it's only a view-specific override. If you want to use a fill that is only available as a model pattern, you'll have to convert it to a drafting pattern. If it's available in a pat file, then you can do that easily.