Does anyone know of a dock for an iPhone that has standard usb data/power, but also a headset jack (not just speaker / headphones output)?
The biggest problem you face here is the fact that you want to use your phone, as a phone, while in a dock.
BluePhone Elite was an option for this (with serious caveats), but it never had terribly good support for the iPhone. Only because of the limitations imposed by Apple. And, apparently BPE is dead now? What a shame. That was a good app for many many other phones.
I'm (unfortunately) very confident in saying that you will have a very hard time finding this dream product of yours, and you're better off simply plugging your own headset into the headphone port, or like balexandre suggested, pair a Bluetooth headset. A product like you want will invariably not pass Apple Certification, which means you will not easily find it in any mainstream store (not only Apple Stores, but BestBuy and it's ilk).
I wonder what is that for, if it's to connect to a stereo, the jack on the Universal Dock is more than enough.
If the idea is to use the phone to answer calls while in the dock, I would strongly suggest a Bluetooth set as that's what they are made for. You will find plenty on the Apple Store as well.