What's the line from Star Wars Episode III when Palpatine tells Vader that Padme is dead? oh yeah - "NOOOOOOOOOOOOOOO!!!!!!"
Ignore the dealer - instead, call your national BMW head office and ask them for an answer
in writing, in case you should need it in the future. Without a written response from BMW, there is no proof that they advised you that commercial car washes are safe. After all, if the canvas top ever becomes damaged, BMW head office will make the call on whether it will be covered by the warranty. A new canvas top for your Z4 is an expensive part by itself, and it is complicated to install and that entire process takes many hours, so the associated labor is also very expensive. Unless you have a few thousand euros / dollars / pounds sterling to burn, you do not want to be told that the top was damaged by the car wash apparatus, and BMW won't cover it.
I've owned an E85 Z4, with a canvas top much like yours, for over ten years and I would
never subject it to a commercial car wash under any circumstances whatsoever. Its one of the reasons my car still gets mistaken for brand new, even today.
And the fun part: