You should be able to go to the catalog in the finder and rename it. Then open Capture One and select it from the Recents List.
I renamed one and it was listed in the top of the Recents List with the new name. C1 seemed to know I had renamed it.
If you want you can test this by creating a new catalog while in C1 and then quit C1, rename the catalog, and then restart C1.
Good Luck.