Okay,
At this time UE not able to give that you directly; But I have Solution For this…,
You Just Want TurnTable Right…,
Use A flat Background Like Matt Pass while You render Your Model.
Then You Can Seperate That Colour as for alpha Using Foundry Nuke Or Adobe After Effect. You can also do This in Photoshop CC.
It is simple process like Rotoscopy for excrate mask or Alpha.