Yea. It's certainly not guaranteed, but so far all the characters that have appeared in the backgrounds are ones that I really didn't expect to be playable in the game in the first place anyways (or, in Nue and Murasa's case, were ones that, while possible, I didn't really mark as particularly probable). Considering that the game has an all-new engine and all-new sprites, it's a pretty safe assumption that the first game in this "series" will have a limited character roster, and thus one likely restricted to the characters relevant to its stated storyline premise. And we already have an intro which seems to be a decently safe assumption as well as to what characters will probably be in, and so far it and the "background assumption" seem to work hand in hand with each other.
With the characters in the intro being very probable and Ichirin confirmed to be in, the odds of Murasa, Nazrin, or Nue being playable in a game that will likely have a limited roster would have been low even if they weren't shown in a background, I think.