The term immigration uses is "Departure Ticket." So yes, your departure ticket will work. There's even signage at immigration in Suvarnabhumi that says that.
Requirements change like a fart in the wind. Recently my local IO started asking me to draw a map of my wife's house. How stupid! They can Google it themselves. Just go with the flow. Nothing you can do about it.
New Update. Just arrived at BKK and foreign passport holders need to show passport, eVisa and arrival ticket. It's on a sign. I would have taken a photo for proof but no photos are allowed. For visa on ARRIVAL you need a DEPARTURE ticket also on a sign. I got in with my oneway ticket. Now sitting in front of 7-11 having a beer and waiting for my flight to Ubon. Cheers!