OK, I tried it - in the middle of a (saved) game.
It worked fine. Prior to this point, I had installed in order:
- CIV IV
- update to v1.61
- BTS
Then, installed:
- Warlords
- patch CIV IV to latest (non-beta) patch.
- patch warlords to latest patch
- patch BTS to 3.13
Started the game back up, and it worked fine. (I love it when this happens.) Some patch fixes were noticeable immediately within the game.