Im assuming you're talking about cutting a hole in where the navigation sits ? I thought about that, some after market radio installer have done it. I just never went through with it because I couldnt find a good gps at the time.. now it might be worth revisiting it.
like getting this piece cut out and fitting the mini in it ?
http://www.ebay.com/itm/BMW-Z4-Navig...e7fb6c&vxp=mtr