Glad you figured it out (the reg key and the .ini file). I thought we had the reg key issue with Civ Complete nailed down, but I have been away from the project for several months. I guess that was just creative misremembering on my part.
I'm sorry, you are correct about putting it into the CustomAssets folder to work with SG games. My bad. I was thinking it would interfere with HoF, which it may still.
When you install BlueMarble (I haven't used it since starting the BUG Mod to avoid conflicts), it puts its art files in the CustomAssets folder if I remember correctly. If you can see the BlueMarble textures when you play the HoF mod, then I expect BUG to conflict with HoF. The reason is that HoF and BUG modify some of the same files.
In that case, you could start your HoF games with the "Lock Modified Assets" option checked. Maybe the HoF crew does that automatically -- I only played one HoF game so far. Actually, since they give you the save file, you have no chance to do that, so hopefully they do and the option is saved in the file.
After working that through, I realize that you should be able to answer the question easily. When you play HoF games, do you see BlueMarble textures? If not, BUG should not conflict with HoF. If so, they may conflict. Sorry I can't give you an absolute answer. You may just have to try it out. Even so, if there is a conflict, you can use Ruff's CustomAssets switcher program.
Edit: If you don't mind, could you possibly post the reg keys that Civ Complete
did create, if any? Thanks in advance!