I'm trying to figure out if a common english usage I really really Verb something is used in Chinese.
Could I say 我非常非常喜欢这首歌？ Could I use this in spoken Chinese? Could it be written this way if it is a more informal context - i.e. a diary?
Sure. You can even repeat it more times (
Like the English usage ("really, really") it isn't something you'd put in formal writings, but its certainly common enough in dialogue or advertisement or blogs/internet posts, etc.
Yes, you can say "非常非常" to emphasize how much you think of something.
It is an informal usage, so of course it is ok to be in an informal context like in a diary, not in a formal paper.