24's rampant use of Canadian actors aside, wasn't it filmed in L.A. just like ENT was? Morrison could have just gone to a different studio in order to appear on the show.
I suppose it's possible. I'm pretty sure Gregory Itzin played Admiral Black while he was playing Charles Logan (when did he have time to grow that beard?), but he wasn't technically a series regular at the time. As I said, if an actor is a contracted regular on the show, the network sometimes vetoes that actor from doing guest spots on shows on other networks. (And yes, there are exceptions. I think Morrison did an episode of "Numbers" while he was on "24"). Again, apart from hair color, though, I don't think he looks ton like James Daly.
I think Morrison would have been a great Flint. I loved him on 24. That soft, smoky voice - pensive and a little melancholy, like Daly.