I have done an implementation for OpenACS 5.1.5, which allows modules like lars-blogger to generate Google Sitemaps.
The infrastructure package can be found at
http://www.clasohm.com/prj/clasohm.com/browser/trunk/packages/google-sitemaps/
The code that generates the sitemap for lars-blogger is in
http://www.clasohm.com/prj/clasohm.com/file/trunk/packages/lars-blogger/tcl/sitemap-procs.tcl
http://www.clasohm.com/prj/clasohm.com/file/trunk/packages/lars-blogger/tcl/sitemap-init.tcl
http://www.clasohm.com/prj/clasohm.com/file/trunk/packages/lars-blogger/www/index.vuh
For this to work, you will also need the directory "google-sitemaps" in the server root, which must be writable by the Web server.
The generated sitemaps can be downloaded at [ad_url]/google-sitemaps/index.xml. See
http://www.clasohm.com/google-sitemaps/index.xml
for an example.
If you want to retrieve a copy of the code, you can do so with Subversion and the URL http://www.clasohm.com/svn/clasohm.com/trunk/