One year ago, Rick Gleitsman (Percy's grandson) emailed me that Sony Japan was about to release a 12-CD box set. This morning I was delighted to learn details of this set from 小野寛, a Japanese fan.
The bonus features of this set include The Great Concert (recorded live in Japan in 1974) plus a 104 page booklet. Details of the set can be found at otonano. The set costs more than US $300 but the music is priceless! Unfortunately, they ship only to Japan addresses. If I find other resellers, I'll post details here. Meanwhile, please enjoy this unboxing video from Sony Music (Japan):