Just to increase the knowledge base, I just designed a solution for my problem.
As mentioned before, I created a custom content type using Dexterity so it would show in the navigation menu. At first I didn’t see any indication of the type of the content but I’ve just found the <a> has a class that distinguishes these static text elements as “contenttype-static_title”. The solution to presenting it as an actual static text lies in the theme. Under the rules for the theme, I placed this code:
<replace css:content=".navTree li a.contenttype-static_title">
<h3><xsl:value-of select="." mode="raw"/></h3>
In this case, I wanted to present the static title as an <h3> but one could replace with anything. And that’s it, the static titles will be shown in the navigation tree right where you place them.
Just to note, I hadn’t noticed any tags in the <a> element but I also had disabled showing icons in the navigation tree. If the class does not show up, maybe it will only when the navigation tree is set to show icons (I don’t actually show them, but it works for me).