It is possible to use macros from output templates directly in your XML comments. This is deactivated by default because usually you don't need it. To activate this option, you can set it manually in project properties. Just open .vsdoc file for the project and change the value of VBdocman_allowMacrosInComments key from 0 to -1 (there is no GUI option).
There is one drawback when macros are activated. Macros have the form: $MACRO-NAME$. If you want to use $ character in your comments, you have to escape it with $$. Otherwise, you'll get compilation error and the documentation won't be generated correctly.