Just to follow up: no luck. I went back to a config.plist from before I installed Catalina, and all the SIMBIOs entries are identical. There were a few settings in the config that were different, such as MatchHeaders and a few other things, and so just for giggles I changed back to those and rebooted. At first I was encouraged that I had to sign into my account all over again (a few times) and thought "Aha!"... but alas... no. Everything still seems to work, but of course I'm still getting the No Updates message. I think I'm hard-wired out of updates at this point. I'll just have to check on my other machine, and download again on this if I find something needs updating.
Very annoying.
Now, however, I have a config.plist for this Hero XI mobo that is in heaven-only-knows what shape. Can you by any chance point me at someone's working config.plist for this, or who to ask? I'd like to know what -should- be enabled/set from someone with a working machine.
Again, thanks for your help and couragement.