Can someone say (or speculate) how this agreement was worded that you mention? Like Mattel asks WB/DC "We hereby ask for your permission to make an exclusive Hal Jordan figure and as to not violoate the definition of exclusive give up all rights to ever use this characters name or likeness in said toy line."
They didn't have rights for the entire DC universe back when they made Hal. I can understand them wanting to make a nice collector's item for the staff at WB, but I think the exclusive packaging and signed cards are enough of a differention from a release version to still feel like it's exclusive if they ever were to release Hal.
I have a feel this issue is really grounded in someone's integrety/stubbornness and because they "said" it would never ever get to the public - rather than based on any sort of economic or business sense.