At the beginning, this is just a Yamaha synth patch I played live. But I copied that in the end section (when the song breaks down for the last time and there's a piano/eery wave deal), and did some things to it in PD.

I created a very simple delay patch, using as its actual basis Travis' same effect-delay sub patch, and I ran each channel of the stereo piano track through that patch individually, dragging the sliders for delay time and feedback gain in realtime while the output was recording. I would create swells using the feedback gain. I did this twice, once for each channel, and then in Ardour, I offset the right channel of the original track by one measure and the left channel of the effected track by one measure. Then I mixed the two together to create the coherent stereo-piano-swelly-swirl goodness.

Piano PD Patch