I haven't heard any Edge-solo piano piece ever, I think. Maybe you got confused and mean the famous "Piano Thing" by Matt Bellamy, Muse's singer?
I'm not sure what he meant with piano "piece" (complete solo or not), but I believe there are more piano parts than you think in U2 songs.
October - Angels Too Tied To The Ground (maybe it's this one) - Disappearing Act (end) - New Years Day - Wave Of Sorrow - Sweetest Thing - Miss Sarajevo - Moment Of Surrender - City Of Blinding Lights - Mercy... To name the first 10 that come to mind.
All have (a bit of) piano in it. Some actually played on piano by Edge live too, so...
And imo first 3 can somehow be called solo pieces.