I have been contemplating the same thing myself as I have a VDO oil pressure,
and a oil temp gauge along with sensors that I got from an Audi 4000.  I sell
these black, abs plastic, one, two or three gauge pods that I think would look
good either on top of the Vanagon instrument cluster or on the side of it.
The plastic is nice and "flowing" looking, leaving a hump for each gauge.
They are from Rocky Mountain Motor Works and sell from them for $15 for the
three gauge pod and $12 for the two guage pod, and one guage pod for $10.  I
can get you a discount on these prices (I have to check tomorrow to see
exactly how much) if you would be interested.
Right now I have my "wet" oil pressure guage attached the the lower left side
of my steering column by tie wraps (doesn't look as bad as it sounds), but I
would much rather have the electric gauges and have them up where I can see
them.  Perhaps you could stick them to the instrument cluster with velcro tape
so they could be removed when you have to remove the cluster cover to change
bulbs or check brake fluid.  I don't think I would mount them using screws as
I hate to do any permanant cutting or drilling in my interior (I just hate
when I get a car that the door panels or dash has been hacked up by the PO).
Ken Wilford
Van-Again
John 3:16