the way i was told to put IPTC metadata to eps is using adobe illustrator, by FILE -> file info, same path as what you do in photoshop for jpg.
but i notice that some eps works and some don't..i am not sure why. it is just disappeared after saving. I found there is too many jobs for that since most agency accept JPG IPTC that uploaded together with eps.
By the way, i think there is a bug that in BIGSTOCK that if you didn't upload JPG with eps, the system may not be able to generate sometimes. 'image being generated' will display instead of the image preview.
Hope that bigstock can work on solutions that read IPTC data from JPG that upload together with eps.