Or maybe I'm mistaken and you just want Photo Mechanic's field names to match those used in iView Media Pro? If so, the field names are not customizable
Sort of ... what I really want to do is to insert {filename} into the IPTC Source field as a "Stationary" item or some other automatic system.
That is what Photo Mechanic excels at. Here is how you do what you're trying to do:
Select your images.
Open the IPTC Stationery Pad (Go to the Image menu and choose IPTC Stationery Pad...)
Find the Source field.
Set the checkbox next to the Source field. (It is called an 'Apply' checkbox, and when it is set to 'on' the field to the right of it will be applied.)
Enter {filename} into the Source text field.
Uncheck any other fields you don't want to apply.
Click on the "Apply Stationery to Selected" button and the filename of each photo will be written into the Source field of each photo.
HTH,
-Kirk