Since I still believe that this film does not take place in just one timeframe, or even just TWO timeframes (I fully expect to see a series of significant events told in a number of settings, including Pre-Cage, Post-Cage, Post-WNMHGB, TOS, post-TOS, and (though it won't be part of the storytelling, it will be the setting from which the story is being TOLD) the post-TNG era.
Technically, there's no reason that Sulu, as captain of Excelsior, couldn't appear, in the appropriate timeframe.
But from a STORYTELLING standpoint... this movie is from Spock's perspective and is ABOUT Kirk. Sulu is a minor tangential character at best. The movie will be better off without him, except as a background feature in the short TOS-era vignette.