Personally I'm not that keen on the positioning, also it may get in a real mess once you have the cables running up to it. That aside, I didn't see anything on there for BMW cars? Z4 specific, I don't think the mirror attaches in that way (but really don't know). The rain sensor sits at the back of the mirror though and you will probably also have some wiring through from the alarm sensor and auto dimming - not sure how easily the mount would fit over those.
Hopefully someone with a little more knowhow will be able to confirm, but those are my immediate thoughts.
Personally I have a vent ment, use by BlackBerry, which is neat, out of the way and the cable is relatively unobtrusive... although it obviously does prevent some heat / cooling loss.