I was the studio producer (Aufnahmeleiter) as well as executive producer for the Steinberg Hindemith recordings. Steinberg certainly heard those as well as the Planets and Zarathustra in quad during playbacks, as we did all our monitoring during the sessions in that mode. I remember that he liked the sound, but wasn't an active participant in managing it at the electrical level in the same way that Stokowski apparently had been.
I think that Pentatone's foray into the DG quad vaults, which I instigated, may have been cut short by its success - - i.e., DG discovering that there was a market for this medium after all, albeit perhaps more on Blu-ray than SACD. But, after the initial instigation, I have not made any specific release suggestions to DG, as they are in a better position than I to determine economic viability of their various quad recordings.
I was surprised to learn of the duttonvocation Fiedler package, and am confused by it, as it seems to contain a lot of tracks which I recorded for DG, mixed in with a lot of tracks that I didn't, and the latter must have come from RCA, excepting that I was unaware of RCA having recorded the Pops in quad before we got started there in 1970. So I'm confused!