ph drop

[Aquattro]

Yes, a volume of salt water, when depleted of alkalinity AND fed a source of acid would theoretically go very low on the pH scale. However, this same volume with an aragonite substrate would still go that low, albeit at a much slower rate.
The rock would act as a buffer as the pH got low enough to initiate dissolution of the CaCO3, but again, the alkalinity would eventually deplete, removing any buffering capacity.
Also, keep in mind that rock and gravel will contribute to this in very different ways based on available surface area. The amount of exposed rock surface is minimal compared to the surface in a sandbed.
Remeber though, that aragonite doesn't dissolve at pH at normal seawater levels.
In theory though, yes, you're experiment would work. I think.