Based on ML lack of any support for the Theater center channel speakers there will likely be a market for a refurbishing service. I don't think I'm that person, however, I'd be willing to inquire some details of how to diy directly from ML if they are willing to play ball since they won't take on the responsibility themselves. I suspect there's a decent market for this with most of these speakers at 20+ years old by this point.That sounds like a winner idea!
Once you know the deflection you can tension the diaphragm to the same deflection to yield the same tension that ML applied. It's better to over-tension rather than under-tension the diaphragm so you want to err on the high side. Also; before tensioning the diaphragm, add some reference marks on the film using a fine tip felt pen.
Then, after the film is tensioned to the determined deflection, you can measure the distance between reference marks (post stretch) to determine the percentage of elongation, and the elongation is the what you would specify in your rebuild guide.
Others could then use your guide to tension their diaphragm using the same thickness of film and stretching it to the specified elongation, which is easy to measure with reference marks and a ruler.
For example; if the elongation is 1%, you would place reference marks 12" apart and then stretch the film until reference marks were 1% farther apart (12 1/8" in this case).