I used the term "flat" in a relative context. Next time you are strolling about a marina with lots of boats sitting on the hard, compare the fwd section hull forms and you will notice that some are less rounded- more flat, particularly toward the bow as some manufacturers use that shape form to gain interior volume.
The hull flex was most noticeable close to the design waterline. We were on a predominantly port tack pounding into a 4-6 ft sea with a stiff breeze for approximately 10 hours. Using a flashlight to cast a shadow across the hull interior, the flex was pronounced and observed by all of us, not just me. As it was close to the fwd bulkhead, I attribute the oil canning to the hull form and structure rather than lack of support evidenced by the proximity of the bulkhead.
This makes a little bit more sense to me. I bet you saw the flex a bit higher than at the design waterline. If you were in 4-6 foot seas beating into a stiff breeze, I bet you were heeling a fair amount, which means the topsides were well into the water and feeling the effects of the sea. I still find it odd that you would get oil canning very close to a bulkhead though, and I still have to question exactly what was going on, but you were there and I certainly was not.
In terms of your point about the flat hull form, it doesn't take much curvature at all to make the structure quite strong, or at least substantially more strong than if it were truly flat.
By the way, I don't think designers use flat underbodies to get more interior volume. They do that for performance and form stability. Indeed, flat bottoms actually decrease usable space below, as you have no bilges for storage, which means you need to get that storage elsewhere. Increasing beam forward gives you more volume, but I don't believe a deeper forefoot does that (though again, I could be wrong).