list the exceptions where you don't want to replace in the rewrite rules before the one that matches the underscore, and finish them with [L], otherwise they'll get replaced too.

[N] means rerun the whole replacing stuff once the line matched... careful with that.

RewriteEngine On
RewriteRule ^tag/?$ /browse.html [L]
RewriteRule ^tag/(.*)$ /mt/mt-search.cgi?tag=$1&blog_id=1&IncludeBlogs=1 [L]
RewriteRule ^mt-static(.*)$ - [L]
RewriteRule ^(.*)_(.*)$ $1-$2 [N]