unlikely...

If the parameter is being passed through a URI segment, the only thing you can really go by is the URI. If you're dealing with a relatively limited number of controllers/methods and the webinar_id is in a consistent location in the URI, it's fairly easy to just check the necessary segments and determine whether the value in segment 3 should be a webinar_id (for example, if the first and second segments match the required values). In the end, though, it probably requires a level of knowledge about the controllers/methods being called that really isn't appropriate for a hook.

Whether there might be some other way to do this without altering the methods themselves really depends on the existing code, but it is unlikely.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide