You could easily install a permanent bourdon tube-type mechanical
guage in the engine compartment.  I think I've seen the ricer crowd
installing these things all over their engines for bling-bling affect.
 I doubt one of those would suffer any undue corrosion problems, and
it would work nicely for quick checks when lifting the lid.

Casey
Olympia, WA
Biodiesel: "I drive in a persistent vegetative state"
'87 300TD intercooler (211k)
'84 300D (211k)
Gashuffer:
'89 Vanagon Wolfsburg Edition (187K)