Isn't this two entirely different things:
a) not taking the inbound flight
b) not taking the first leg of the ticket?

Not taking the first leg of the ticket does definitely not work. However,
I discarded the return flights 3 times in the past 5 years (once even on advice of the KL ticket desk in AMS), always with my FB number in the booking, and I never had any problem.

And yet, I would do it only in emergency situations, you never know if you might get into trouble (i.e. are charged for a one-way flight in the end).