If you really want to know the coolant temp as well as a number of other parameters, there is a way to find out, although it's rather awkward. There is a "hidden menu" that tells this stuff, as outlined briefly in the "Displays, Indicators and Controls" pdf. The following pic is from there. There's also a discussion of using this menu on the e60 forum:
http://forums.e60.net/index.php?showtopic=21759&st=15