Thanks Ken. Great news; I pushed my test server up to BE12 with all the nice liveupdate patches, smacked it into Exchange (2007) GRT and let it run for a couple days.
I'm getting a glorious collection of IMG folders, exactly as 11D created.
The document you linked to says that BE12 "by default" deletes IMG media every time a GRT-enabled backup runs. It's not. I'm thrilled... and not migrating any of my B2D customers to 12 until I get this worked out.
By the way, there's another article in Symantec's database about GRT recommendations. They're identical to 11D recommendations, which again, are cumbersome and undesirable.
I'll now begin the dig to see if there's a setting that's stuck from my upgrade from 11D to 12 that's left me with non-default behaviour. I'll probably end up calling support and losing a few hours of my life.
Thanks though, I hadn't seen that particular article to know that theoretically this is supposed to work.