The attached workflow shows how it is possible to use the File generator plugin to create a text at run-time without writing to a physical file.
The important part is to enable "Return file content?" and to not specify a target file path.
In this example an xml is generated with variable parts specified at run time.
The file content is fully customizable, so this would work with any sort of text file, not just xml.
At run time, the generated text will be available as output