When I got to my office in the US yesterday, there was a pleasant surprise waiting for me. The generous donor of the Convex C1 XP had found another box of tapes he once used with the machine. He did not recall what was on these tapes, but had them sent to my office.
When I opened the box, and started looking at the labels on the tapes, I realized that these tapes contain dumps of all the filesystems that were once on the C1. That means, that once I get a suitable SMD disk for the C1 - or manage to get my SMD Disk Emulator working, I can restore these filesystems and hopefully boot the C1 off it.