Crazy thought here: What if DS is always masked in JL, has a significant, but small, part in the film, AND they are planning on him playing a much larger part in the solo Batman film, BUT have not yet settled on casting him for the Bat solo.
So, with it being a smaller part, and if he remains masked, what if, and just hang with me here for a minute, what if they have Joe playing the part in JL, with intent to recast for the solo Bat film?
I know, it sounds crazy, but if you consider that the part in JL is likely not that large, probably not really focused on building his character, AND Snyder really liked Joe, and wants to work with him, why not do something crazy like this with a smaller part, for fun?
Hey, we know Snyder's idea of "fun" with characters is a bit out there to begin with, so I don't think I'd really put it past him.