I told Cassandra that she shouldn't become the Divine, told Leliana that she should, I supported the independence of mages and somehow Cassandra became the Divine. I mean, I'm sure there's a reason somewhere, the way it turned out just went completely against what I would have expected.
To my knowledge I never got a war table operation to support either one though. Could just be that I, in a moment of weakness, found myself more bored with the war table than usual. Perhaps I just arbitrarily sent people to do whatever operatons my cursor happened to be closest to at the time without paying much attention and, as they say, **** happened. But I doubt it.
You get war table operations for Cassandra or Vivienne if you talk to them about becoming divine and say you'll support them. I've never seen more than one operation in a game, so it may be getting one locks the other(s) out. I've also not seen an operation to support Leliana.
All in all, I really like how your choices throughout the game influence who becomes divine in the end. I recall that the suggestion that things should more often work out that way, without a big one-time choice at the end, came up when talking about endings in general in the context of ME3. It seems Bioware has taken that one up, both in this outcome and the epilogue line about the general style of your Inquisition, and I appreciate it. It feels more natural.
My impression is that if you roleplay consistently, you'll get what you want. For some people, the mechanic seems cryptic though. At some time, it would be nice to know the details, just so we know there isn't some odd bug in the calculation.