You might be interested in this thread from 2019 here on HN as well, I linked to a line of work on constrained sequence generation from Sony CSL that is pretty nifty https://news.ycombinator.com/item?id=21334639

As an aside - do you have any academic references for masking / constraining based next step transitions? It's a trick I've been messing with for years, but I probably picked this up from some older poetry generation work. Haven't been able to find any direct reference for simple 1 step masking, let alone more involved things like constraining based on the unrolled, modified markov sequence or things bigger than 1 step.