Surprisingly, Cobie Smulders says that she is free to appear in S.H.I.E.L.D. because there is no conflict with her commitment to HIMYM. She also said that there have been discussions about that happening.
One can only hope that this means it's possible that Samuel L. Jackson can make cameo appearances as well. I would much rather see Fury stride onto the screen and start kicking ass than have Hill on the show.