I built a house in a flood zone. First, Randy's right - find a new agent. Tell them to read the Flood Insurance Agent's manual. It's crystal clear that it can be sold based on estimated elevation and estimated value.
Joker: If you delay the purchase of the insurance until the slab is poured, you right about having to track it (more work and risk), PLUS getting the borrower to cooperate after loan closing is not always easy, PLUS there may be a 30 day delay in getting the insurance (spending on when it is applied for).
THUS, just get it before loan closing and avoid of all of this hassle. Again, if the agent would simply read their manual, they'll see it can be done and it's not that hard.