One of the reasons for converting DPack from a collection of add-ins to a single VSIP package was to gain access to text markers. Numbered Bookmarks feature utilizes that with text marker being bookmark images shown in the Visual Studio gutter.
In his post Alex shows how to use text markers with add-ins, i.e. w/o VSIP. Keep in mind, that while you can get to the markers already exposed by VS, AFAIK there is no way to create brand new markers with add-ins. You’d still need VSIP for that.
Here’s his Text Markers article.
Sergey @ USysWare