I just turned dir_index OFF with tune2fs. Now the directory order is the same as the inode order.
This makes the order of files predictable and in fact turns out to solve my problem.
With dir_index turned OFF on that filesystem, when a copy is made to another directory (even from Windows on a Samba share) the alphanumeric order is preserved. I will just ask the workstation operators to copy the PS files to a new folder when they are all ready. Distiller is watching that folder and will process the files in the normal way, using the rundirex file.
This solution is even better than the initial situation: since we can now predict the order in which the pages will be processed, we can manipulate the order at will by doing multi-phased copies to the folder, in any order we want, instead of being limited to the alphanumeric one provided by NTFS :-)
So "dir_index ON" (and my ignorance of the inner workings of EXT3) was to blame for this confusion, from the beginning!
What a trip this was (sometimes in circles)! Thank you very much to all who contributed! Great community!