What I'm saying is that

  [...]
  ((input-pending-p)
   nil)
  ((<= seconds 0)
   (or nodisp (redisplay)))
  [...]

would become

  [...]
  ((input-pending-p)
   nil)
  ((<= seconds 0)
   (to-be-determined-call-to-timer-check)
   (or nodisp (redisplay)))
  [...]

The idea is to compensate for input-pending-p no longer calling
timer_check so as sit-for will behave the same.