I can envision an exposure meter that would be equipped to handle the situation.
A real-time continuous photometer, similar to those built-in to the shutter mechanism of "Auto-exposure" cameras, with a processor capable of being programmed to the reciprocity curve of the film. Could be servo-coupled to the shutter of LF cameras.

Now to make it pocket-size, and figure a way to produce the thing for $3.98...