beardedbruce, if there really is any reason to deny passengers access to their liquids and gels during the flight -- which I doubt --, underseat storage could be banned except for small things like computer cases, purses, etc., and those could be screened for the dreaded Metamucil, Old Spice, Right Guard, lipstick, and (shudder) Diet Coke.
No liquids or gels under the seat, but you could still take one small carry-on bag, including your toiletries, and store it overhead in a locked bin.

But right now, the US still allows baby formula and prescreption meds in liquid or gel form to be carried on. One of my meds is in a cream form in a small tub. Why would it be any harder for a terrorist to put contraband liquids or gels into a prescription bottle or tub like that than into an over-the-counter one?