In a nutshell - I would like a button somewhere on the ribbon (custom action?) that, when clicked, will form a string which includes the current web's ID and the list item ID.
How do I go about achieving this?
This code will give you current selected items:
and this will give you ID of current SPWeb:
When creating Ribbon buttons I usually put these in a separate file and call them from Command, like this (mylib.js):
and then call this from the definition of your custom Ribbon button (Elements.xml):