tpl_product_info_default.php would add a link to all product pages without touching the individual products ...

However if not all products have a contract then you want setup based on some type of condition on whether to show or not show ...

NOTE: If you fool with the READONLY attributes ... there is a way to use this for links ... but if you edit them after adding them it will break the php code or link code in most cases ... so lots of backups ...

Advantage of the READONLY Option Name Type is that it is not added to the order ...