The lost motion assembly on the y8 is different than the z6. Not sure about the rocker arms though...
I can't imagine them being all that different... When I was installing my cam a while ago, I failed to notice that one of the rocker arms on my z6 head had rotated slightly until as i was tightening the rocker assembly down, and I heard a crack. I broke one of the vtec rockers, and since I couldn't get my hands on a z6 head at the time, and I had a y8, I used the rocker assembly/lost motion assembly from that... No problems so far.
Edit: Knock on wood!