Announcements are made directing PS passengers to use the economy lavs, and that is the expectation (unlike some other airlines that have dedicated PE lavs). It's unlikely an FA would stop you if you tried to use a D1 lav, just as they usually don't stop domestic economy passengers from using the F lav, but up to you how you feel about doing that. On the A332 you would have to walk all the way through D1 to reach a lav at the front; on some other aircraft like the 764 or 359 with lav(s) at the back of D1 it might be a bit less conspicuous to slip in.