Well the idea isn't totally shot, just need to get your own microcontroller hooked up to the CAN-BUS network and capture the transmission of that particular message from the OBDII port/AlfaOBD, then add a button to your micro that regurgitates that 2WD message into the network and then you have push-button 2WD on-demand. If you are good you could read the 4x4 status and have an LED indicator as well.
This is one of those things that would take a lot of initial work getting yourself onto the correct internal bus network and sifting through probably thousands of transmitted messages to find the 2WD one, but once that hard part was over you could start a small company and churn out plug-in OBDII modules with a button and an LED on them, with about $5 worth of electronics inside and all it would do is inject the 2WD message into the bus. You could sell hundreds, probably thousands of those units at $100 each and we'd buy them up like snack cakes.
I'm pretty sure this is what the ESS defeat modules for the other FCA cars are doing, a simple module plugged into the OBDII port that detects when the car starts and immediately fires the "Disable ESS" message into the network. Combine those two things (ESS disable and push-button 2WD mode) and you'd have a popular device for sale for sure.
Any of you medium-sized aftermarket companies listening here? Madness autoworks, where you at?