I've kust made tests with gocr (a OCR command-line linux software) and it proves to be safe, i.e. if it fails to detect a text, you see some nonsense collection of symbols.
That part is definitely NOT safe in the context of spamassassin... Nonsense looks a lot like bugs in spam mailers, and very little like legitimate email to SA.
If nothing else, consider the tripwire rules, which look for letter combinations that don't exist in normal English...
