Upon looking closely at your photos - Ruffus, your string pull will not work with a broken or displaced cable, will it?
Your string mechanism will work if you've lost the key, or left it under the seat, because your string is attached to a WORKING cable - not a broken one.
For those who don't understand - the mechanism we're looking at is the key-lock/cable-connection, with the cable leading back to the latch under the rear of the seat. This trick relies on the cable being unbroken, and cable-ends being fully attached at both ends.
Often - an emergency seat latch device is attached close to the closing jaws on the latch itself, or to the jaws themselves - and works to open the seat even if the cable is broken, frozen or displaced - or if the key is lost or not working.
But this string will open the seat - if the cable is good. I do not think Ruffus meant for this project to be taken as a fix for a broken cable.
Stig