I don’t know if I understand the special distinction you’ve described, of an internal link only being capable of pointing to a formal chapter or section title—though I suppose if your binder outline precisely describes formal headings in the output, that conflation would be appropriate. Otherwise you can link to anything in the binder, it doesn’t have to be something that prints a heading, and you would address it normally by its binder title when typing in the wiki links.
If by this distinction you mean linking to text inside binder items, then this how-to should help.