The tool of 'changing hand' in keyboard music is, as far as I know, a common character all over in the existing music literature.
Maybe to be added best under 'Text'.
It's just a single character 'L', easily to be composed by underscore (_) and stile (|) and its upside down. Except, that Text isn't bound to a specific note or position in the score, so, one has to correct when e.g. inserting or deleteing measures
Don't use the text tool for that. As miker suggests, use an expression (it will follow the music even if the layout is changed). In this case, as the symbol includes several characters in specific positions, you may have to create a shape expression.