todo-mode-line-function

todo-mode-line-function is a variable defined in `todo-mode.el'.
Its value is

todo-mode-line-control

• This variable may be risky if used as a file-local variable.

Documentation:
Function that returns a mode line control for Todo mode buffers.
The function expects one argument holding the name of the current
todo category. The resulting control becomes the local value of
`mode-line-buffer-identification' in each Todo mode buffer.

You can customize this variable.