Never tried it myself, although I was planning to add a new Flavor once the expansion comes out, since I've been using FLAVOR_RELIGION for a lot of my Mythology content and presumably that flavor's going to be used for a lot of expansion stuff. From what I've seen, it's one of the few non-item tables (as in, not Units, Buildings, Technologies, etc.) that doesn't rely on an intermediate Lua table like TerrainTypes or something similar. So, the only problem I'd think you'd have is that the existing Leaders wouldn't have a Flavor bias for your new Flavor; the biases default to zero, presumably, but 5 is the actual baseline value, so you'd want to add an entry for each leader, including any DLC ones.