Well, it's official. I received the answer from Omega, and they do not retrofit "1st generation" cal 8500 movements with the newer silicon balance spring. The answer also came fairly quickly, given the festive holidays, which was nice. Here is the response:
Dear Mr ChronoScotSo, despite my slight misgivings about the thickness, looks for the moment like I will have to pick-up a new PO to get an Omega with a silicon balance spring...
We thank you for your e-mail and are pleased to learn that you are the proud owner of an OMEGA Seamaster Aqua Terra, calibre 8500. We would not miss this opportunity to thank you for the confidence you have placed in our brand.
Further to your enquiry, please be informed that we do not retrofit earlier versions of calibre 8500 with a silicon balance spring. Such an upgrade will not be possible in the future either.
We trust that you enjoy wearing your fine OMEGA timepiece and are convinced that it will give you excellent service for many years to come.
Your OMEGA Team.
Hopefully this helps others for future reference.