I think Codex swapping is an underused thing to do with marines. While some players might get annoyed if you do it incessantly, I think if the representations are clear it opens up some great "counts as" possiblities. Want to field your army in a special jump assault? BA codex. The last stand of your famed heros? SW codex. etc.Thanks for all the help guys, you've all got some rep as a thank you
I've decided to go for the Night Lords and use the BA codex primarily (maybe use the CSM codex ocassionally, just for a change). It's sounds like great fun, thanks for all the thoughts and suggestions it's all been really helpful.
Have fun with the Night Lords!