I would imagine that anyone is now free to produce Treemonisha provided they do not imitate the Houston-Schuller DG material. Several revivals of Joplin's original score, with varying orchestration, have and are being produced.
The popularity of "The Sting" (produced in 1973) coincides with the early work on the Houston Opera production (stage design, etc. was initiated late in 1973). It undoubtedly revived interest in Joplin's music and also added to the interest in the Houston production- without this interest, it is doubtful that Deutsche Gramaphone would have recorded (and released in 1976) a production by an opera company little known to the world at large.
The audience for "The Sting", the general public, would have gone to sleep at any opera performance. Initial sales of the LP package were good but soon decreased. DG has kept it in the catalogue on cd, but it is purchased only by a small specialized audience.
Perhaps the hope for a more "authentic" version rests with a crop of new musicians such as Nevilla Ottley and small, intimate productions. Certainly the interest in the black community is low; revival of a vehicle that illustrates the superstitions and beliefs of the blacks of the late 19th C. and early 20th C. would be regarded unfavorably by many.