The problem is that there's no way - besides scripting - to tell the game that you want to use a different head for a player character. For NPCs you have the CRE file. That file may specify a body *and* a head. But the same entries in the player's CRE file are ignored. That's because the CRE files are static, while the player's appearance is decided when you start a new game.
And: Bioware has decided - for the sake of simplicity, I suppose - that player characters may not have mixed models. If you pick a complete model - the Watcher e.g., or the Ravager - you should be fine. But if you do want to use a mixed model, you must do it the hard way. That means calling SetAppearance after every Transformation e.g., reverting the character to its original appearance before the in-game movies play, etc. etc.
To get (a human readable version of) the heads.2da you should download the lot of them from papagamer.com. The link is somewhere in this thread, I believe. You can also find it in the Credits
section of my mod's web site.
Sorry that I don't have better news.