It would definitely be useful to be able to insert an ad-hoc line of text in the turn order. You could use it for all sorts of purposes: track effect durations, mark start-of-round, keep track of time, set reminders to do something, etc. In particular it would be VERY useful to be able to have some way of inline editing remaining-duration for effects.
In order to maximize flexibility and minimize UI overhead, a simple editable text line (perhaps with macro-evaluations?) would probably be the best first-cut. A GM-only vs. viewable-by-all distinction might be useful as well.
Come to think of it... if you could just "Add Turn" for Text drawing elements (rather than that menu item being restricted to tokens only) from the main battle grid area, that would probably be good enough. They're editable, and you can already control their visibility. Bonus points if you make the display text in the Turn Order tracker editable and mirror the text back-and-forth.
You can sort of achieve that already using blank tokens, but just allowing text drawing elements to have their turns added directly might be a cleaner solution. It would be fewer clicks and dialog-views to edit that text ad-hoc than it is to edit the name of a token.