Made some minor progress again yesterday and today.
I did a search/replace in a text editor to change all instances that pointed to their servers back to localhost, did a bit of other tidying, and imported the SQL file minus the users into my v2.2 instance. I'm now attempting to upgrade, but it keeps aborting because some table already exists that shouldn't, or vice versa and I have to fix that and start the upgrade right from the beginning again.
They need their data migrated because they want to import it into a different LMS that only supports MBZ and Common Cartridge files, and they don't want to recreate all their courses from scratch in their new LMS if they can import what they've already got. As you note, none of the conversion tools are much use.
Thanks again for your help. I'll have another go at this to see what I might still be doing wrong.