I just added a new feature to the "Show Actions" commands. The html windows generated now contain a checkbox, using which you can toggle whether the corresponding action is marked as completed or not. Each time you click the checkbox it writes on the corresponding file. It hasn't been checked much, so, you know, make backups and all that. Any errors encountered will show up in Console.app. You will need to switch away and back to textmate to see the changes in the gtd files.
Haris