Simply using a field PM sees and other programs don't is naive. Others may use PM as well and other programs may also read the fields or develop it later. Your secret is safe only if you protect it with either coding or not sharing it with others instead of hoping they don't find out. Or maybe the easiest way is to tell everyone so noone cares...
You may use codes in the IPTC say liba libb libc or anything else. Nobody would understand except you.
If you put / signs before and after it, you also can use a code replacement file to decode it for you but make sure not to change any IPTC info tag color class or star rating and only add the replacement file to the list after saving your codes into the images. This way only the code is stored but the replaced text is displayed in your PM.
If your images are in RAW format and you don't add IPTC/XMP to the image files but store them in XMP sidecar files, then you can keep all the IPTC fields for yourself if you don't send the XMP file along the image.
Another way is to record the client/library names into a WAV file with the same name as the image. The sound file appears in PM as a loudpeaker icon and is played when you click it. If you don't send it along the image noone hears it.
PM also has some kind of Job/Client management but I never used it so I don't know how it works.
I like your idea of using codes, but without the replacement delimiters. The original poster could come up with sufficiently obscure code names for the different destinations and then use replacement delimiters only in the Info Text panels/tooltips. Thus when viewed only on his system the codes would turn into human-readable names.
So to do this create a Code Replacement file containing codes and replacements like this:
liba Client name A
libb Client name B
libc Client name C
Then in the IPTC for something like Object Name enter: liba
And then change the Info Text (Edit->Set Info Text...) like so:
Client: \{object}\
And when you view the image in the Preview window, it will reveal which client it was submitted to. If you submit a single image to more than one client you'll have to have additional permutations so you can have unique codes:
libab Client Name A, Client Name B
libac Client Name A, Client Name C
libbc Client Name B, Client Name C
libabc Client Name A, Client Name B, Client Name C
This would quickly get out of hand if you had many different destinations for your photos. But if you only have three then this would work pretty well.
Let me know if you have any questions.
-Kirk
