Not that familiar with Constanten solo work but do know he collaborated with/joined the Henry Kaiser Band in the late 80s/early 90s. They used to do Dark Star nightly I think.
Would also say that I think you're missing a lot of great stuff by the pre-retirement Grateful Dead if you only concentrate on the Constanten period. I think a lot of the jamming from the year after he left (Feb 70)
is far trippier than even the stuff he was involved with. May 70 seems to be pretty consistently high for one.
The period 68-74 to me is the high, Constanten was only onboard throughout late 68 and 69. He'd guested a few times before that too but was only a member for a year and a bit.
Most of the freeflight material escapes the boundaries of whiteman's blues and some of the playing inside even the cowboy songs escapes it too. I always think of that freeflight material as some of the closest anybody in the US got to matching the better krautrock.
Stevo