Using P2 to bring out AGND is not a good solution. There is an internal 8K resistance between the internal AGND and P2. This will increase the source resistance of this AGND and any current sourced or sunk from P2 from an external signal will result in the shifting of AGND. The best method is to use the Analog output buffer and bring out the AGND using the RefMux.
You can also bring out AGND using the testmux by writing to the ACBxxCR2 register. In this method, you can bring out AGND from a CT block which already used by another resource like a PGA, INSAMP etc. Following articles describe this method.
Thanks Ganesh. That's a good technique to know!
BTW, the first of the two links didn't work (the one to cypress.com).