Nigelh,
I have a fair amount of NVG time (and before that PNG, passive night goggles, when the use of night vision aids in the RAF was in its infancy). I was a military QHI / NVG instructor, and later a civilian NVG instructor (at a combined search and rescue and police special ops/ support role unit). I have seen totally non-compatible, "cobbled together" and correctly designed and fully compatible cockpit lightings.
In what role (i.e. what type of operation) do you propose using them? Are you normally operating single pilot or two pilot operations?
I have to say that I too have some misgivings about the concept of simply buying a set of previous generation NVGs, even for "only" the co-pilot's use, especially in a non-NVG compatible cockpit. The civilian unit SOPs I operated under forbade mixed NVG / non NVG crew operations in the way you describe.