Thanks Redwing, I've looked at those threads, but I haven't found a solution to my problem there.
I did see that the label (if that's the right word) for "Involved People List" was given as "TIPL" in the wishlist topic on regular tags, so I tried using that to create my custom tag, but still nothing comes through.
I suspect the problem lies with MediaMonkey not exporting the tag content - so I'm hoping someone else has had this problem and found out how to fix it......