RonW, the first method did not work, it does not ask me for a password but it exits silently instead. I couldn't find whether pp "extractor" code can be user-specific and couldn't see where in PAR modules the extractor code is. Any pointers on that welcome.
For the second method you suggested, it should work but I am in Linux and still looking for 7z's "extractor.exe" linux-equivalent. But it looks like it is going to fly... alternatively, following your model, I can always make a self-extracting archive linux-style, I am the only one needing to run this, as I do not trust this code with hosting companies. But if a hosting company has a wizard who could sneak onto the temp extracted PAR files, then let it be, it's not the end of the world.