Matt Long has already made an appearance in a comic book adaptation (playing a young Johnny Blaze in Ghost Rider), but we're as happy to forget that as he surely is. Since then, he's made frequent appearances as a journeyman TV actor. From Private Practice to Lucky 7, Long has found plenty of female fans along the way.
How well the actor would fit Snyder's planned take on an older Robin/Nightwing depends on the overall tone sought for Ben Affleck's Batman. We know this dark knight will be older and more battle-hardened, and if that's the case, a Dick Grayson who's happy to lighten the mood from time to time makes perfect sense.
Long's signature aloof personality would cover those bases well, without straying too far into a 'jokester.' Comic book fans know that Dick Grayson is a complicated character - being raised under Bruce Wayne's wing, but certainly not in his image - but we're confident Long could pull it off.