I can understand the concern with maintaining the value of a rare regulator. Dr. Miller's and Duckbill's information shows that diving this regulator may detract from its collectable value, which I would take into consideration.
Saying that, my Hydro Twin has been in the water many times, and is a very nice regulator. A New England Diver's decal on it would not have affected whether I would dive the regulator, as I don't collect them to just look at them or as an investment. I collect them to understand their engineering concepts, and to use them. My Hydro Twin is a regulator I bought from a guy in Winston, Oregon in the 1980s from his garage for $25. I was able to completely disassemble it, and had to improvise a different gasket between the first and second stage as the original one disintegrated. All the other parts, even the second stage seat, are still original and work. I did lubricate the O-ring for the seat. I did change the hose/mouthpiece system, and have the original mouthpiece. But I now am using a Hope-Page mouthpiece with Superflex hoses on it. I did take some measurements of the original mouthpiece non-return wagon wheels, and since they have only four "spokes" the surface area was between a Healthways mouthpiece and a USD/Voit.
But realize that my reason for collecting may be different from others, and act according to your own interests.
Now, concerning your question about the diaphragm, unless it has a hole in it, the original diaphragm is probably still good. It is rubberized nylon, and will last almost forever. You can take it out, put a coating of silicone grease on it for a few days, and it should be good as new. Be sure to wipe it down very well, so that it is not slippery when you put it back on the regulator though.
Below is a photo from a few years ago of me using my Sportsways Hydro Twin with a Dacor Nautilus system.
SeaRat