Can you drill holes in the jib tracks between the original maunfature's holes to allow for more jib sheeting positions?
No.
The Cape 31 International One Design Class Rules are closed class rules (Part II preamble). The World Sailing Equipment Rules of Sailing define closed class rules as (ERS C.2.2): “Class rules where anything not specifically permitted by the class rules is prohibited.”
As the class rules do not specifically refer to the addition of holes to the jib tracks any addition of holes to the jib tracks is prohibited.
Andy Wibroe
International Class Measurer