Newish P12 owner here, but I was getting some interesting sounds from the feedback section, by modulating the feedback amount, with an aux 3/4 envelope, the feedback amount decays quicker than the note, or is just there for some spice on the attack.
Karplus strong works by feeding a short click into a short, resonant delay, to make it ring. The VCA is after the feedback section, but you can make a click before the feedback section by using an Aux Envelope (3 or 4) to modulate the Osc. level. You set the Osc. level knob to zero, and then the amount of the aux envelope modulating the Osc. level to however loud you want the click to be.
You get different sounds by using a burst of noise, or a high tone, into the feedback section.
Modulating the individual Oscillator levels with the Aux envelopes is a nice feature of the P12, in general.