My understanding of pre-population with the reg hack, is that 'all' messages that fit the criteria (X number of messages or X number of days), will be sent to the device. Doesn't matter if the messages have or have not been sent prior.
I believe the setting that makes this happen is ForceMsgPrepopOnActivation - 1
http://www.blackberryforums.com/bes-...tml#post751856
One caveat I suppose, is if the user has moved the messages out of the InBox into a folder that is not being redirected, I would guess those messages will not be pre-populated.