If I did decide to update the interior (which it looks like I am now), I know i'd be tempted to do something with the dash. Something similar to what you see in a AC Cobra and/or replica. Granted, its a lot of work though. And more spondoolies.
Taking the dash out is relatively easy, have done quite a few (not on a Z3 mind). I am not bad at hacking into instrument clusters to tap into after market gauges and dials. I don't know at this stage what I can achieve with the stock instrument cluster.
I bought one for £20 today off ebay (as I don't have mine at present), and took a closer look ...
With the front plastic removed, there is a possibility it could sit flush behind a custom dash (ply with vinyl for example), and the rings get fixed in place from the front (with perspex/glass inserted).
The fuel gauge and temp gauge can be hidden. I can attempt to connect Lucas 2" gauges to their connector at the back. Something like these ...
I cannot see why the same cannot be done for the warning indicator strip that runs across the bottom of the Z3 cluster. Connect to each respective lamp, and run it to one of those Lucas instrument clusters.
So much for not doing the interior.