The pain receptors your thinking of aren't ones for physical damage, they are chemical and strain receptors. Ever got a stomach ache? Your stomach hasn't been physically hurt, it's just the chemical receptors translating bad levels of ceertain chemicals in the stomach as pain. Strain receptors are in muscles and measure how much strain they are generating.
Edit: Hehehe nevermind, strain receptors also feel stretch and they are also in many organs(brain and pancreas exempted).
I think the type of internal/organ pain people are searching for is called sympathetic pain. The internal organs have sympathetic and parasympathetic connections to the brain as part of the autonomous nervous system (for organ control). Their "quality" of pain tends to be more generalised in location and duration when compared to nociceptor pain. It also leads to the phenomenon of referred pain, where the pain manifests in seemingly unrelated other parts of the body.
I'm pretty sure this hasn't been included in the game body models, so adding pain nerves to the organs seems like a reasonable thing to do if you want to simulate that pain. My adventurers feel so much pain already, I doubt they will grumble at a bit more.
Good job on the tweaks OP, I'd like to see this make its way into the game if it works out well.