It's certainly ok when we convert to BTS for the BTS version of the mod. Those who have BTS could play the BTS units, and there would be no need to upload them. As an example, while we use swordsman graphics in EMA, the actual graphic is already on the computer of the person using the mod, and is not uploaded as part of the mod... the xml files simply access graphics already installed with the game.
Similarly for BTS. If the mod was converted to BTS we could use those units since they would not be included in the mod, per se, just accessed through xml. The graphics would already be in place.
What we really can't do is upload BTS graphics as part of a CIV IV vanilla mod. I'm certain this is frowned upon. That being said, I'm not certain how reskins of BTS units, for example, should be viewed.
Likely a search of the forum might provide some info as this has likely been discussed previously.
Sorry, have to go for now.