I have my own theory as to why the word came to be related to writing. Pothooks would have been made by the local blacksmith - each of whom would have had his own design. So much as one person's writing varied from another so did pothooks.
"The pothooks straggled a little wildly across the page, as if written too hurriedly, on a pad balanced on someone’s knee." (Mary Stewart - "Touch not the Cat" 1976)