Not to throw a wrench in things, but...

As a Y passenger I absolutely will use the F lav under the following circumstances...
1. FA never mentioned line about using your own cabin's lav
2. Curtain has not been pulled shut
3. I'm in EC (or first few rows) with Y lav occupied (esp if lined up)

I'm not going to walk the full length of the plane to stand in line when the F lav is unoccupied. If it were the middle of the night on a long flight, I would be more hesitant to use F lav.

I would never want to wake an F passenger who rightfully deserves a bit more privacy.