Lith film negative. Enlarged or in contact. If you want it in the image area do the exposure separate from image. Used to be common in labs.
This is usually how this would be done. If enlarged, a second enlarger could be set up to project the signature. If contact printed, it could be done most easily with a contact proofer that had a box with individually switchable bulbs and a timer, and it would make the exposure when the lid was closed.