While answering the usual questions about appearing in something as massive as a Marvel Studios production in a video interview with IGN, Ming-Na Wen (who will play Melinda May in the show) inadvertently reveals that former Firefly crew member Ron Glass (Shepard Book) will be making an appearance in Agents Of S.H.I.E.L.D. Skip to around the 48 second point for the reveal.
So, Ron Glass will be a part of the show. How big a role he'll have an who he'll be playing is anyone's guess...so speculate away!