Luthor hasn't always been lily-white. The DCAU Luthor is modelled on Telly Savalas, who was Greek, and has a fairly swarthy complexion.
Besides, ethnicity shouldn't be a dealbreaker. We've had the half-Japanese Dean Cain as Superman, the half-Chinese Kristin Kreuk as Lana Lang, Sam Jones III as Pete Ross, Michael Clarke Duncan as the Kingpin (a particularly apposite example), Billy Dee Williams as Harvey Dent, Dania Ramirez as Callisto... heck, we even had Eartha Kitt as Catwoman in the late '60s. (And you could include Halle Berry, though her "Catwoman" was a totally different character, and really, why would you want to?)
Oh, I agree entirely. In fact, I usually like it when productions cast against typical expectations and really do think it keeps things fresh, especially in a long-running property. Obviously, there are some cases where that wouldn't work, but more often than not, no one really thinks about doing it until it is already done.
I was actually thinking of Michael Clarke Duncan when I suggested Tahir.