Ok, I am going to answer my own question...
Since I could not find any way to do this, I wrote an Apple Script (
https://transienteye.wordpress.com/capture-one-resources) which will check a selection of images to find those that are offline and copy the paths of any offline images to the clipboard.
I think that I should be able to do this using the "Folders" tool, which normally marks folders/disks which are offline. However: I have at least one catalogue in which there are offline images (due to changing the disk they are stored on) yet nothing shows up in the folders tool as
missing. Using the script I can at least find the offline images and relocate them manually (a tedious task, as it can only be done one image at a time - although at least I can tell if they are all done now!).
It would be possible to automate the correction of the image file locations, except that the path attribute of the image in CO11.2 appears to be read-only...