There are a few things that can be changed (text keys, and a few others), but most of the database is cached when the mod is loaded, before LUA kicks on, so not many changes are possible.
The modiki has some information that might be helpful - especially in the Unit Instance Methods:
http://modiki.civfanatics.com/index.php?title=Unit_(Civ5_Type)
Look closer at the "S" part - the "Set" methods to get an idea what's possible, e.g. SetBaseCombatStrength...
Unfortunately, most of these methods only impact a particular unit instance - not for the entire class or type. Even if you found a method to alter something you want, you'd have to iterate through all the player's units - but that's not that hard.
You may want to look at creating a few different promotions to provide the different capabilities you want to use, then use LUA to award the different promotion depending on the "Raging Barb" setting is.