Let make sure I understand, you are using a plane-mounted NMO (or mag-mount, lip mount, it doesn't matter really) with a SO239 adapter to a short coax feedline to a bar-mount antenna?
Electrically it will show conductivity but you can't necessarily assume it to be a good or universal ground. Reason for this is the radiating element itself needs to couple to the return, so the distance and coax impact the characteristic of this. The physical distance and material used between the antenna and the mount and around the ground plane aren't random.
Short answer would be that it will work but there is a performance impact. How much? Probably not enough to notice normally but you are losing efficiency, so it will ultimately be non-ideal. The nice thing about the NMO mount is it's a uniform impedance across it, so the discontinuity will be primarily in the SO239 & PL259 parts.
Also, yes, the sheet metal of the truck will still act like a ground plane. The difference is that you have to visualize the extra mount and coax as an impedance between the hot and return. You will be forcing RF current on the shield, which is a restriction but no worse really than any other feedline.
Would a sketch be useful? What will actually happen is the short patch cable will act as a small antenna most likely. I'll have to mull it over, but you might want to put a choke on the outside, although if it's short I don't know that it will matter much either way.
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