"I wonder if Mallozzi was thinking that far ahead, all the way from mid-season 1, or if he got the idea to use Sarah for this later."
I had years to develop the show and the individual character backstories and arcs while working Stargate so, yes, much is seeded early through subtle hints that pay off much later down the line (hopefully with the full five seasons I have planned). In the case of Sarah, check out the end of Episode 9 (season 1). After Sarah has been declared lost, we go to a quick scene of FIVE looking down at the stasis pod, furrowing her brow and cocking her head as though getting a thought. It's a throw away scene that is quickly forgotten - until this season when Sarah makes her return. There is (will be) a lot of that throughout the series run.