My immediate thought is a script attached to each NPC that checks the relevant journal entry, disables the NPC if it's below the "launched" value, or enables them once you accept the quest.
Something like this.
CODE
Begin IDENTIFIER_script
if ( Journal "some_journal_id" >= some_value )
Enable
else
Disable
endif
End IDENTIFIER_script
Where IDENTIFIER, some_journal_id and some_value are placeholders.
So what should happen is that when you enter the cell with the NPCs is that unless you have accepted the quest, they won't appear. Once you do, they appear. I'm sure someone can spot flaws in my scripting and correct them accordingly.