Hi Mr. Brainz, thanks for your comment.
I’d certainly prefer to ‘extend’ these functions instead of hacking like this – I’ll look into hooks and see if I can do this here. MultiThumbnail looks fine, but it would mean that the user will have to upload duplicate images which may get confusing and this will further confuse the custom theme code for displaying the appropriate thumb in the correct location. My method may be messy but the end result is simpler for the user I think.