Thanks for all the responses so far.
In a practice, Les is correct that I have yet "over-expose" the situation. Over exposure isn't really the concern, it's how to achieve ANY exposure in the most economical way that's the question.
Also, I have bracketed the shot -- both by going out on consecutive days and on one memorable occasion, by shooting the P6x7, the 4x5 and 5x7 at the same time.
The end result is that I've gotten the shot I was after, but I'm still not sure I did so in the most efficient (and repeatable) way given whatever it is that's going on during reciprocity failure.
As for metering the scene, that's impossible. The lighting conditions are just too dim. For most of the type of things I like to shoot I've learned not to even bother taking a meter out with me.
Anyway, thank you all for your thoughts and if any more occur to you, please post.