I use a build facet to run the generated java classes from my language. This produces further text files in a sub directory of the classes_gen directory. This works ok so far. But there is no integration of these files into the GUI since MPS does not know anything about them.
In MPS 2.5 it was possible to use the logger methods directly in order to create messages with links to files. This seems not to be possible anymore in MPS 3.x. But I'm sure there are now better ways to handle such a use case. But unfortunately I don't know them, yet.