Ok a update now cause I just tested google now while screen off and after saying ok google it does not respond it does only if the screen is awake. I had no idea it can do that also. So the result is that is working with lollipop but not while screen is asleep. I am Greek so sorry If I messed up and got you confused with the latest results I will try to create a tasker profile if it's possible that it will trigger the google now while screen off. I will update if I have any success.
Strange cause I did a little search and I found that to make google now listen while screen is locked you have to enable the option at google settings language and input voice but there I have only 2 options for listen while google app and any screen. No lock screen options for me as they claim. Maybe it's removed from lollipop?