On the smallpicture-user list, Andy DeSoto asks a great question about the built-in slogan macro. He wants to know where it's defined. That isn't the issue, if you don't like the way it works, you can override it, by writing your own version of the macro, and putting it in your cmsPrefs.opml file.
If you look in my cmsPrefs.opml file, which you can view on this page, you'll see that I've defined several macros. If you define your own #macros table, and put a script in there called slogan and have it return the empty string, you'll basically void the built-in routine.
Here's a screen shot of the macros table with this new version of the slogan macro.
Here's the docs page on defining your own macros.
And to answer the original question -- the built-in macros really are built-in, they're not in a place that you can edit them.