.htaccess mod_rewrite IP-based redirect: how to redirect all traffic to a specific subdirectory, except my IP?

You should be able to redirect with the following:

RewriteCond %{REMOTE_HOST} !^123\.456\.789
RewriteCond %{REQUEST_URI} !^/blog/?
RewriteCond %{REQUEST_URI} /(.*)$
RewriteRule (.*) /blog [R=301,L]

You should create a page that handles the specific redirects, and then edit your htaccess file to be something like this:

Options +FollowSymlinks
RewriteEngine on
#not your IP
RewriteCond %{REMOTE_HOST} !^123\.123\.123\.123
#make sure the rule allows everyone to access the redirect page
RewriteCond %{REQUEST_URI} !/redirect_page\.html$
#Send them to the redirect page
RewriteRule \.html$ /redirect_page.html [R=302,L]

The rediret_page.html can have either js or server-side redirection that handles where they end up, but it will force everyone who has not come on your IP address through a specific page that handles redirection.