I have a MK1 LS400 with 228,000 miles. The bushes are pretty much dead and I have been looking for a cost-effective replacement.
If you need help with development I might be prepared to lend my car on condition that I got it back in one piece and that this help was reflected in the price of the bushes.
I did contact Daizen asking if they did rear UCA and LCA but never heard back from them.
I would have thought there would be some demand for a complete kit. Maybe not so much over here but there should be for LS400's in the US and Celsior's in Oz.
Re the harsh ride, I spoke to Carl from lexls.com about that. He feels that the ride isn't unduly harsh with poly bushes and retains most of the quality ride.