Hey Martin -
MDX isn't the easiest language to pick up and the basic expression samples we provide don't do more than scratch the surface of what MDX is capable of. You're one of the lucky (?) few actually interested in learning / writing this stuff!!
In your shoes, I'd probably head over to Microsoft's SSAS discussion forums and see if you can't get an MDX guru to give you a push in the right direction. Good luck! MDX is not for the faint of heart!
I know.........and I'm not sure I'm going down that road but still it would be a nice thing to have on your CV