So you're basically looking for a self-grounding normally open 'Hobbs' switch, would work the opposite of how a normal oil pressure unit works ...
They were used as the brake light switch for some odd vehicles during the 60's - Jeeps (
) IIRC used such a switch.
I've worked on vehicles back into the 30's that also used pressure switches on the brake line(s) to activate the lights - nothing new here.
Problem is relying on the brake plumbing as a ground in most of these - works great new, but as things rust ...
Is there any reason you could wire one side to a known ground and then the item/circuit needed 'grounded' to the other lug?
These switches were used by lots of OEM stuff for different applications - GM used two different functioning Hobbs units in the Turbo Regals to monitor and control the engine functions both on and off boost. While these are not common parts (in both known or available terms), they are very reliable.
There are also plenty of Hobbs type switches used in industrial applications that may also work - just check a Honeywell or other suppliers catalog for applications.
You're local 'Industrial Controls' dealer might have just what you're looking for.