File:Mousekeys+key.png

Summary
Jagex Mod AC confirms that the + key is legal. The picture shows the original post of the player-created thread asking if the + key is legal, and below is Mod AC's final response.

(Note: this post of Mod AC's was actually on page 4 of the thread, but is shown below the OP so that people can see the question and answer. Mod AC did make another post on this thread suggesting that the + key might not be allowed, but this other post was on page 1, and the later post shown in the picture reflects his FINAL answer. And as he said, he "had some interesting debate," implying that some of the J Mods talked about the issue and then came to the conclusion. Mod AC's first post should not be taken as evidence that the + key is not allowed)