Even though the soon-to-come DasBlog 1.8 will include support for Custom Macros, you CAN get custom functionality now with this existing macro:
This will load the specified ASCX control into the page at this location. The code-behind can be in another (non-DasBlog) assembly.
This won't give you access to the DasBlog DataService, that's what the new Custom Macro support does. Still, pretty slick, and a good reminder from Michael Earls.