I am wondering if it would be possible to connect a resistive touch panel (manufactured with a 30-pin connector) to the iPad through the 30-pin connection and use that for input?
No chance I'm afraid, the limiting factor is software more than anything though, it's possible that an App could be written to talk to said accessory, but using it outside of that App would not be possible unless you are talking about a highly modified jailbroken solution.
Unless you already have a device in mind that exists which you actually want to use, I can't imagine anyone creating one, and if they did not by the dock connector, but more likely over a custom Bluetooth protocol which is allowed (see the i-Cade games cabinet for example of a custom controller using Bluetooth that only works with supported apps).
You may be better mentioning what you wish to achieve to allow alternate answers, as I am almost positive that this method will never fly.
it is possible to connect an external touchscreen capable of sending input to iphone: http://www.macstories.net/iphone/the-external-touchscreen-that-can-control-an-iphone/