They're not exactly made by TUSA or Scubapro, they're made by Tabata, the same company that makes SP and TUSA regs. TUSA is the U.S. branch of Tabata, I think. Anyhow, Halcyon regs appear to be clones of SP designs that are particularly popular with cave divers; the MK25 and G250V initially but they might have expanded their line by now. I guess they're for cave divers that can't stand the thought of using anything without a nice blue H.
I don't this for a fact but I suspect that SP parts will work fine in Halcyon regs, and there is a large grey market for SP parts. So they are available, its just a matter of how and where.
In Europe, any SP dealer will sell parts to divers, it's a law. It's only in the U.S. (and I suppose Canada) that the dumb we-want-to-save-you-from-yourself policies exist.