Update: I figured out a way to do this with some of the utilities built-in to 10.4, although the PDFpen folks seem to have a good third-party solution as well.
- perl driving ghostscript, well GNU software installed via fink.
- a combination of a Folder Action script, and a customized version of the Combine PDF Automator action, which basically is a python script driving the CoreGraphics framework, join.py
/System/Library/Automator/Combine PDF Pages.action/Contents/Resources/join.py
in the shuffle function I made a simple change to add the second PDF pages in reverse order.
My plan is to set this up as a Folder Action using the convert postscript to PDF script as a template. The script should do nothing when there is only one file in the folder, but when the second file is added, move them to a "Processed folder" to empty the watched folder, and generate the new PDF with the odd pages PDF shuffled with the even pages PDF in reverse order. I will assume the order by date will put the odd pages PDF first and the even pages PDF second, since you would scan the pages face up first, then flip them to scan face down. I'll post back the scripts when I finish it in case anyone else might be interested. This will be a handy turn-key solution for dual-sided scanning with a one-sided scanner. I am kind of surprised that this particular solution hasn't been posted somewhere already, or if it is I just couldn't find it.