Major Description
The class org.apache.pdfbox.multipdf.Overlay has no possibility to perform the overlay only in memory (using a custom OutputStream). It is currently fixed to work with files only.
A suggestion would be to add an overload public void overlay(Map<Integer, String> specificPageOverlayFile, OutputStream outStream) throws IOException which uses the outStream if supplied and falls back to outputFilename if null is passed (would be an easy solution)