I don't see any problem on allowing PCs to create warforged, as long as it starts as a 1st-level character and do not automatically obey the order of the creator.
Most PCs can produce a character of PC race without specific feat anyway. With right mate, a human PC can produce a human (or half-human), right? It takes about 15 years to get an adult one, though.
There are even spells to create sentient undead immediately. Many of the undead creatures created by Create Undead and Create Greater Undead spells can be playable as a PC if DM allows to do so.
Never the "only one" right?
That is what I'm talking about - creating a race not creating a character from an established race.