I had a thought about another way to add more tasks without having to set up tasks for reassigning the buttons, if the menu idea doesn't work.
You could add a long-press on the select button to switch to a different set of button assignments (or cycle through multiple lists. This would take a lot of tweaking in the phone app though, I think.
Also, FYI re: the menu idea. the Watchnote app guys have somehow figured out a way to send a big chunk of text without much of a delay - 3 seconds - which is only 1 second longer than it usually takes for PebbleTasker to load the 3-task list.