Curiously, a "sequel" (B.C.: A Special Christmas) didn't show up until 1981. It featured the voices of Bob Elliott and Ray Goulding this time. Johnny Hart is given full story credit here (he splits it with Jack Caprio on the Thanksgiving special), although the story is surprisingly secular given Hart's later penchant for proselytizing in his strip and how religious the Peanuts special was. The only real religious reference is a set of three men on camels walking off towards the North Star at the very end.
As near as I can tell, neither B.C. cartoon has seen wide syndication, nor have they been released on any form of home video. Evidently, they just didn't charm viewers in quite the same way Schulz's characters did. YouTube, however, does have the Christmas special as a single video, and it looks like there's enough clips of the Thanksgiving one to see that in it's entirety as well.