I've still got some work to do on the filter to ensure that the generated signature doesn't include excessively long lines and I also want to improve the templates to allow a form of conditional operation.

Something like

{ Fax Number %Fax% }

meaning that if the "Fax" field is empty in the database (i.e. the particular user doesn't have a Fax) then the "Fax Number" string is also omitted from the generated signature for that user.