I know this is something we already knew, but Ausiello was at the Fringe Premiere party and Lance Reddick confirmed that he would appear but never said how many times.
Ausiello also spoke to JJ Abrams who said he would be interested in directing an episode before the series ended.
Below is Ausiello's post about Reddick and I posted the video of the interviews in the Media Mentions section here:
JJ Abrams Interested in Directing an Episode
Question: Any spoilers about the Lost characters we already know and care about? Enough about the newbies. -- Mary
Ausiello: Does Lance Reddick count? If so, this week's Ausiello TV's got scoop on whether his regular gig on Fringe precludes him from returning to Lost this season.