If you'd like a leader without any text you can use the Text tool to create one, but there's a bit of a workaround- until I find out if Revit can support Unicode Format codes within standard text via keyboard entry...
1. Create an aligned dimension and edit the text within the dimension.
2. Choose the "Replace Text" option.
3. Right mouse click inside the text field and choose "Show Unicode Control Characters..."
3. Right mouse click again and choose "Insert Unicode control characters..." and the select the "LRM/Left-to-Right" control code.
4. Right mouse click inside the text field and select "Select All".
5. CTRLl+C to copyclip the Unicode Format Control LRM into your windows clipboard.
6. Start a text with leader and when the text field opens, CTRL+V to paste the LRM into your text.
7. Click Modify to finish the command.
You should now have a leader without any visible or printing characters or text associated with it!
You can add additional leaders to the "text object" or remove them but, you cannot edit the text in the note once youv'e done this as it's not selectable. The same is true of a dimension that you may have replaced text with only the Unicode character.