Can't you plug your headphones into the mixer along with everything else?
Had a setup like that with my MP11SE : line-out from the Kawai and stereo out from the iPad into a Behringer Q502 mixer, and then to a pair of iLoud Multimedia speakers. Had my headphones connected to the mixer as well (mixer was on the music rest, next to the ipad).
But I don't like the fact that the headphone wire runs all the way under the DP to the music rest, gets in the way of my left arm when playing.
That's why I want to use the headphone jack front left. I used the mixer at first because plugging the iPad straight into the line-in of the MP11SE produced some hiss, which I didn't like either. But for some reason, the hiss disappeared almost completely when I tried it again a couple of days ago. Go figure.
Anyway, the D1 has no line-in, so the mixer would be the only option, again causing the left arm wire problem, messing with my head again (I know, I know)...