Minibb get indexed pretty quickly anyways so not sure there is much need for a sitemap but I have got in the habit of using them in Google's webmaster tools.
Dunno if there is an addon for a sitemap? Couldn't find one anyways so I knocked one up myself. Dead simple coding really. Creates an ordered txt file of topics. Text files can be submitted to Google, Bing etc.
3 files in all
. One to create the sitemap, another to hold the data and lastly a file to ping the search engines.
1st file - sitemap.txt save as sitemap.php
2nd file - create a file called sitemap.txt and make sure it is writeable - chmod of 666 works fine on my server.
3rd file - sitemapping.php - save as sitemapping.php
Save all 3 in the root of your domain.Easy admin links to update the sitemap file - ping etc
I doctored the templates admin_panel.html file like provided.
You can now create a sitemap whenever you want, view it for any errors and then ping the search engines.
It doesn't cover forum titles or comments and so on but they could easily be added. I only use one forum so topic titles are enough for me. Anyways Google will still find your forums and comments from viewing the topic pages.
There is a limit of 50,000 urls with Google webmaster tools but again that isn't a problem as I have more elaborate sitemap.php files that create as many sitemaps as needed - sitemap1 - 2 and so on whilst keeping the urls below 50,000. Some of my sites have over 100,000 urls and I have to create 3 sitemaps. All 3 can be created at the same time quite easily. If you have this problem then ask for more info.Sitemap and rich urls?
Google won't use the created urls but your minibb rich urls if your site is using such, otherwise google will use the created urls.