Okay here is my attempt to put the audio overlay patch into a Hijack v300 kernel. I also built it with IrDA. I haven't tested this kernel out (I'm at work now and can't put it on my Empeg), but I built it from a regular Hijack v300 kernel tree. The audio overlay patch didn't go in very easily, but I think I did it right.. Give it a shot.

I also threw together a pcmplay binary which should theoretically take advantage of this (opens /dev/audio with O_SYNC.) Source here. Also untested.