Yeah SMG was done with Buffy when she was on Buffy! There were so many stories about her exhaustion from the physical demands of it. I remember hearing an interview with either her or Alyson, I can't remember at the moment, saying Sarah was so worn out, the writers wrote that episode where Buffy's turned into a rat so she could just do voice-overs and not have to fight for a bit (and that was only in season 2!). So yeah, my wish for a cameo is probably a pipe dream.
But I would take Alyson any day! Honestly, I'd be happy to see any of them come back for even just a scene or two in the reboot. They probably couldn't play themselves, but it'd still be neat!