Well then it looks like I was quite wrong and these are real Kenner prototypes after all. My apologies for my scepticism and especially for doubting the clearly excellent memory of Mini99!
If these were indeed sculpted by a Kenner employee, and especially if done so 'unofficially', perhaps that explains why they aren't more well-known. It would certainly explain why they aren't more polished. And if they were made with the Cantina set in mind (i.e. circa 1978/9), perhaps that also explains why they were kit-bashed from original figures rather than new sculpts, and also why Pin-Head Han was used rather than Balloon head.
This is a perfect example of why I love this hobby! There's always something new to learn
...Now if you'll excuse me, I must retrieve that three pound note from Donald's Trump's wig before he rides off with it on that unicorn