Strangely enough, wandering on in the Internet I casually found this promising project lead by Rebecca Benefiel: The Ancient Graffiti Project
"The website provides a search engine for locating and studying graffiti of the early Roman empire from the cities of Pompeii and Herculaneum.
Ancient graffiti, inscriptions that have been incised or scratched into wall-plaster, comprise a special branch of epigraphy. They differ from inscriptions on stone in several respects. An inscription on stone may be commemorative, dedicatory, sacred (to name just a few classes of inscription), but in almost all cases forethought has gone into the preparation of the text and the inscribed monument. Graffiti, by contrast, are more often the result of spontaneous composition and are the handwritten creation of the “man on the street.” Since graffiti are scratched into friable wall-plaster, they are more easily perishable, but when they do survive they are almost always found in-situ, unlike many stone inscriptions that have survived to the present day through re-use".