It is fairly common to have the “—” or “___” commands in Markdown supported tools. If unfamiliar, this would create a full-width horizontal line. It would be great to have this included in Dynalist.
<hr> is definitely useful when composing traditional keyboards. Outlines in Dynalist are not traditional, linear documents though. For that reason, I suggest using an empty item with a color label as the horizontal row.
When you put a horizontal row, I understand it as it’s separating two sections. I think it makes sense that these two sections should be in different items, thus making an empty line a good way to separate them.
Or did you want to put only <hr> in an empty to separate two items?