I have developped for our own needs a small utility named pdfBooklet, and since it was useful for us, I thought it could be useful for others and placed it on SourceForge. This is an alpha version because the gui is unfinished. After that I discovered your program and I thought : why develop in parallel a small brother ? Why not include in your program which is already popular what makes PdfBooklet special ? Because PdfBooklet has some features I never found elsewhere.
Practically, you have already all the difficult mechanic. What has to be added to give a full control on booklets printing is just mathematics and could be easily included. You have here a model which has been tested on books of 600 pages, and I think the model is now quite robust. I could indicate you what is really needed and what is not. and after that it would be no longer necessary that I continue the development of PdfBooklet and you would give your users the power of professional binding.
Tell me if you are interested and I can give some more detailed informations.
P.S. : I cannot help for programming, I develop in PHP and Python only, I don't know anything about Java.