In reply to:
now my problem is that when I enable that, it seems to clobber webalizer.
That is the result of the .htaccess file contents that wordpress uses. The default .htaccess rules included in the .htaccess file for WordPress produces that error for pages that are *not* part of the WordPress installation. Your new "http://www.<domain>/webalizer, or http://<domain>/webalizer (makes no difference if you have the domains set to use "both" www and without www), is *unknown" to Wordpress.
In reply to:
I might have to do it by hand in order to get the exact right behavior I'm looking for
This is probably true, though you should be able to edit the .htaccess file without any problem (*Your* user should own that file). What you need to do it "combine" the lines in the .htaccess file that allow the password protection with the lines that handle the WordPress re-write rules *and* modify those re-write rules in order to allow the server to display http://<domain>/webalizer. A good guide to doing this to display http://www.<domain>/stats (which is the default DH stats package, Analog) can be found in the Dreamhost WIki article on making stats available with .htaccess. While the examples use other programs and the "stats" directory, the concept is the same for webalizer and WordPress - just study those examples and modify your .htaccess file accordingly.
Similar, and additional information, with other examples can be found in the Dreamhost wiki article on Mod_rewrite. Just take a look at these two articles, you .htaccess file(s) and make the required adjustments; you should be good to go. Good Luck!
--rlparker