With the decommissioning of my old computer, I really took notice for the first time of a strange limitation in WHS - there's no way to export an image of one of your computer's backups to a compressed folder.
I'm officially retiring Tombstone, as you already know, and I'll want to remove it from the "Computers & Backup" tab in the not-too-distant future because I'm a neat freak and because I hate having yellow things in my Home Network Status screen, even if I can check "ignore this issue" to get my thing back to green.
So, with some google-fu and a lot of time, I managed to mount, export and compress Tombstone's final backup, which now sits on my desktop as a 275GB .wim file. That seems a little small, but it'll have to do until I can take a few days and try to restore it. I'm sure I'll learn something interesting during that process, but at the moment, I feel as if the peace of mind I've gained by having the data there is good enough. Even if it's not restorable, I'm sure I can mount the image as a drive to get at anything I may need.
Anyway, this post isn't a how-to, just a note on the fact that it's do-able, and probably worth it.