Due to the age of these cars now most things anyone wants to do has been done before.
Sadly the owners have either moved onto the E89 or G29, the car has been moved on, restored to factory condition, crashed, died, scrapped or something else so don't expect too many replies if you post onto the older posts.
Fitted an electric oil pressure guage to my 1.9 Z3 some time back, mounted mine in a pod attched to the windscreen pillar. Could be done on a Z4 in the same manner, do like the ashtray space used in the example posted earlier, but needs a glance down to read guages fitted there.