I tried to create the Install.img diskette on my NT system. All the other diskettes that I created were unreadable by NT (understandable since they were now an AIX file system).
  The floppy was still readable by NT. After much impassioned sobbing, Rick Ekblaw had me send the base install files (all 19) to him.

Rick Ekblaw pulls up a chair and pontificates with:
  If anyone else would like to make a "fixed" Install diskette, here's how I did it:

    First, note that the existing Install diskette image has PC-DOS 3.3 data in the first 8 sectors (4096 bytes).  Looking at my AIX 1.2.1 Install diskette, I can see that the Install diskette is supposed to be AIX bootable.
    So, using your favorite binary editor (I used UltraEdit-32 from TuCows), delete the first 4096 bytes (000-FFF hex) from the Install image file, then insert the first 4096 bytes from the Boot_1 image file.  Save the Install image file, use your favorite disk image software (I used DSK4WIN from dvalot.free.fr) to create the fixed Install diskette, then follow the bring-up instructions on Louis' AIX PS/2 web page to get to the point where the machine wants you to start feeding it BOS diskettes.

  I did my testing on a 9590-DLG machine equipped with 16MB of RAM, an XGA-2 adapter and a single 540MB SCSI hard drive (Spock Prime SCSI adapter).  During the initial installation phase, there is only a single XGA-2 choice (with 8517 monitor).
   I was using a 9525 monitor, so no problem there.  I decided that the hostname of aixps2 had no character, so I christened my machine "roadkill".  Interestingly enough, AIX 1.3 creates a filesystem with your hostname -- so don't choose a name that you don't want other people to see.