Evince (and others) needs the ability to perform manual duplexing for people who want to save paper when printing. 

All that is required is adding an option to print odd only or even only pages. It is a very simple feature to write and could add a lot of functionality.

This is not good enough (odd/even pages).  You should add "Manual Duplex" as a
option.  First, print odd pages (1,3,5). Then you popup a dialog "put the pages
back into the printer".  Then, you logically (but not physically) ROTATE 180 degrees and print even pages IN REVERSE ORDER (6,4,2).  Like Windows/XP does.


For example from a print driver (and no GUI, which is why I want this, and so
do my users)... 

http://foo2zjs.rkkda.com/
File foo2xqx.c


        if (Duplex == DMDUPLEX_MANUALLONG && (PageNum & 1) == 0)
            rotate_bytes_180(buf, buf + bpl * h - 1, Mirror4);

        if ((PageNum & 1) == 0 && EvenPages)
        {
            SeekRec[SeekIndex].b = ftell(EvenPages);
            cmyk_page(buf, w, h, EvenPages);
            SeekRec[SeekIndex].e = ftell(EvenPages);
            debug(1, "CMYK Page: %d     %ld     %ld\n",
            PageNum, SeekRec[SeekIndex].b, SeekRec[SeekIndex].e);
            SeekIndex++;
        }
        else
            cmyk_page(buf, w, h, ofp);


and:

    if (EvenPages)
    {
        DWORD   pause;

        // Handle odd page count
        if ( (PageNum & 1) == 1)
        {
            SeekRec[SeekIndex].b = ftell(EvenPages);
            blank_page(EvenPages);
            SeekRec[SeekIndex].e = ftell(EvenPages);
            debug(1, "Blank Page: %d    %ld     %ld\n",
            PageNum, SeekRec[SeekIndex].b, SeekRec[SeekIndex].e);
            SeekIndex++;
        }

        /*
         *  Manual Pause
         */
        // Write even pages in reverse order
        for (i = SeekIndex-1; i >= 0; --i)
        {
            debug(1, "EvenPage: %d      %ld     %ld     %ld\n",
                i, SeekRec[i].b, SeekRec[i].e, SeekRec[i].pause);
            fseek(EvenPages, SeekRec[i].pause, 0L);
            if (i == SeekIndex-1)
                pause = be32(2);
            else
                pause = be32(3);
            fwrite(&pause, 1, sizeof(DWORD), EvenPages);

            fseek(EvenPages, SeekRec[i].b, 0L);
            for (j = 0; j < (SeekRec[i].e - SeekRec[i].b); ++j)
                putc(getc(EvenPages), stdout);
        }
        fclose(EvenPages);