BBear, are you asking about Rear (or Trailing) Curtain Sync where, say during a 2 second exposure, the flash will fire at the very end? If so, I believe you need to use an external flash to control that aspect.
Unfortunately, there's no way to get around the 2 flashes of P-TTL using only the onboard flash.
(not to hijack this thread) but also on this subject: For some people, their reflexes are fast enough to blink very quickly right after the first flash, therefore getting caught with lazy eyes in the photo. On another forum, there was a guy who ran some timing test for the lag between the two flashes. Some camera systems are quick enough to beat even the fastest human reflexes, and others are so slow they give quick people time for a full blink. Pentax seems to fall somewhere in the middle I think. I was actually very suprised to only find limited Pentax-related discussion on this issue because me, my son, and both of my parents get caught greater than 98% of the time with our eyes half closed!
The only work-around with onboard flash is using the 2-second delay with places the first flash at the begining and the final photo flash at the very end of the 2 seconds. Glad we are a minority. While it's called lazy eye....our eyes aren't actually lazy at all, quite the opposite in fact!
-jeff