Sigh.

Looked like a great solution, except it generates the same OS error as trying to backup the key in the SSRS Config manager.

I really do think one of the things that got locked down / changed for the recent security audit is the culprit...

Once again, Lowell, thanks.

Now for some good news (for me at least.) Turns out on the QA server (where I've been trying these tips) there's a backup of the key (for QA, but to simplify the Devs life I'm hoping / betting it's the same between QA and Prod) sitting on the filesystem! So I've got that out to the new server, and am working on getting things configured.