this means that I can instruct from my app Bridge to inject Metadata to my files
What I need is a library/routine to call that inject (reading or writing) metadata to different kind of images (EPS, TIFF, JPEG, PSD) without "opening" them visually in Photoshop.
Mark has covered most of the details. It all depends if you want the same metadata to be added to a particular type of file or custum metadata for each file. It may be possible to use Cron to schedule a run of a script. This may require a hot folder where the new documents are to be placed so that the metadata it updated and files moved to somewhere else. If it is the same data, Exiftool would be the best approach.