It should be possible to specify that files are always included in the exported IDE-specific project file, independent of whether or not they are compiled. Consider this IntroJucer project:
Aside from the problem of specifying that .cpp files should not be compiled in a particular IDE, it is not possible to specify that the .mm files should be included in the project. This is the desired output:
Although this is not a big deal in the example provided (the .mm files just include the .cpp) it is a larger problem for xCode. Since the .mm will be included in the Xcode project instead of the .cpp, find in files from the IDE will not know about the .cpp sources.