> If we did this _now_ we could, perhaps, reuse the existing syscall
   > number,  since AFAIK poll(2) has never been in an official NetBSD release.
   
   Yes.  The only conflict is call gate for ABI with some other OS that
   does poll with which you want binary compatability.  Solaris might
   be one example.  It really depends on whether execution classes imply
   system call interfaces, or not (they should, meaning no conflict).

netbsd uses separate a system call table for each emulation
(what you call an `execution class').