Temorary Ballast for Utility Category C.G. Question

(Note:  I have cross-posted this question in the “Aircraft and Equipment” section in an attempt to reach a larger audience)

In 18 years of instructing, I haven't had this issue arise before…

I have a client who would like to use his 1958 C-172 for commercial flight training. The “Owner's Manual” and cockpit placard stipulate that commercial maneuvers - chandelles, lazy eights, accelerated stalls, steep turns, etc. can only be conducted in this aircraft using the utility category weight and C.G. limits. The client is heavy.  In order to meet the C.G. requirements in the normal category, he has 58 lb of removable weight (containers of “kitty litter” and water) located in the baggage compartment, which he removes when flying with rear seat passengers.    However, the placards and Owner's Manual state that no baggage in the baggage area AND no baggageor passengers in the rear seat are allowed in the utility category.  My question is:

Would plastic containers of sand or water, clearly labeled “TEMPORARY BALLAST” be considered “baggage” if stowed in the baggage compartment or rear seat?  I'd prefer using the rear seat even though more weight would be needed because we could use the seat belts to secure it – there are no tie-down anchor points in the baggage compartment and I'm uncomfortable “jury-rigging” restraints there.  My initial determination was that the placard / Owner's Manual language would preclude using ballast, but the client would like independent confirmation.  The local FSDO declined to state an opinion…

Has anyone encountered this problem?  If so, how did you deal with it?

W.E.Dobson CFI/CFII (AOPA #01405812)