If the aim of this is to fit a sealed or better ball joint then fitting a dropper and a Metro Joint is not the best approach.
I've bought many sets of the Japanese sealed types and they are good. He also has an 'OEM' type of joint which unlike every other Joint I've bought in recent times, is actually good, at least equal to, if not, better than OEM Ball Joints.
I did speak a while back with one of the guys from Mini Spares regarding better ball joints and possibly offering these (or their own) sealed units and he was adamant that they wouldn't sell one. I don't run a shop, advertise or try to sell anything these days, but I've sold over 8 sets and have others waiting. I'm sure with the type of exposure Mini Spares has, they would sell well.
It was Just a theory as to why a dropper and a metro joint wasnt on the market, understandable its not the best approach. Intresting to know you've spoken to Mini spares on the subject though.