Collect sum of EMI radiation level as a detector of how many pins in a printer head that was involved in a specific point of a paper is old and relative simple technology, not worth any research. Matemathical anlayze of recoverable information content have also been done 30 years ago, at least for the english language. With a bit more modern EMI collecting and processing technology can result be improved but most results in this area are classified.
An old dot matrix printer do nowadays seldom deliver information worth complicated analyzing. Install a backdoor at the victims mailserver is much simpler, have I heard. :roll: