Update field via url
There's the Prepopulate module


  The Prepopulate module allows fields in most forms to be pre-populated from the $_REQUEST variable.
  
  For example, the following URL,
  
  http://www.example.com/node/add/blog?edit[title]=this is the title
  
  will automatically fill the Title field on a new blog post with the words "this is the title". Any field can be prepopulated this way, including taxonomy and CCK fields. You can prepopulate more than one field at a time as well.


It's only in dev for Drupal 7 but I've used it in a couple of projects and I haven't had any major problems. If it doesn't quite match your use case you could always take a look at the code, it shouldn't be that tricky to adapt for other purposes.