Here is a photo from another X5 owner of an installation he did in the ashtray. Note that its just barely side enough to house a head unit, but it may or may not be wide enough for the Empeg tray. The thing is, I'm pretty sure this guy just mounted the faceplate here, and the actual head unit body is somewhere else behind the dash. I seem to recall that when I spoke with him before, but I'm still awaiting another response.

Also note that the location where you had originall suggested for the stereo is actually all entirely climate controls on the Nav version also (you can see better from this angle).