Is there a library just to generate Scala code regardless of the source language? Basically passing the AST
Do you have any specific languages in mind? You could use the javaparser AST and scalagen's serializer, but that will only work if your source language has a java-like structure and you write a parser that dumps the same AST structure.
Out of scope
Sorry for the delayed response. I was thinking about using to generate Scala code for RAML clients.