I'm way behind in answering these posts.
The 6th and 7th positions can be shown by adding 1 and 2 respectively as a second cell to the 5th position sign (TN or Spec.Sym. entry required). 6th position is (13, 1) and 7th is (13, 12). I haven't seen this in any code; it is just something somebody recommended to me when I did a trombone book once. It's simpler that the string position marks. If anyone has proposed a marking for the trigger I haven't run into it yet. Thank heaven it is not usual to find position markings in normal trombone music.