Yeah, ISO files lack support for saving metadata/tags internally. However, you could try enabling sidecar support which will work for ISO files.
MC's Options > General > Importing & Tagging > set Sidecar tagging mode: to Save in sidecar if internal tagging is not supported then click on Create external sidecar files for media types: and select Audio.
What this does is for filetypes that MC can't directly write metadata/tags to, like ISO files, it creates a file in the same directory as the ISO file(s) which acts as a "sidecar" saving the metadata/tags there instead.