A prior thread confirms much of what I said above. @kayceesrk’s response basically answered my question:
(I guess this is a 2nd [or, Nth] example of where having a way to register C threads with other domains would be very useful! 
)
There’s a workaround linked to in that thread, but there’s also forking going on there, and it appears to be limited to using a single domain (perfectly reasonable, but less than what I’m aiming for here, i.e. pairing Java/systhreads 1:1 with OCaml domains).