I am sure I will continue playing the gu zheng; it's a special instrument and of course I have fond memories associated with it. I actually have 2 of them, a large one with silk-wound strings and a smaller steel-stringed one that is more of a beginner's instrument. Funny that you should mention the dan bau. I have one of those, too; very tough to play--all harmonics!
Zhang Yan has only 2 CDs I'm aware of; one was released in Taiwan and the other in Hong Kong. The first one features more solo performances and the second one is orchestral and features a chromatic "double zheng" that was built for her. I don't know if they are still available.
Yes, I've considered the guqin. Very cool sounding. Thought it was a bit tough for me to me to tackle. No frets--only dots on the fingerboard and a lot of glissandi. I used to have an LP by David Liang Ming.
Yes, I play gu zheng on track 6 of 'Temple..'--an extended improv that Robert somehow coaxed into shape. I met Robert at a Jorge Reyes concert in 1994; I was introduced to him by Stephen Hill of HOS. As for your question about Robert's hand injury, I don't think it has completely healed, but he has made adjustments as to what he can play and has even built new flutes to accommodate his playing limitations.