Something like this:
RewriteEngine On
RewriteOptions Inherit
RewriteCond %{REQUEST_URI} ^/wp-content/.*
RewriteCond %{REQUEST_URI} !.+\.(jpe?g|gif|png|css|js)$
RewriteRule ^/(.*)$ - [F]
I'm not at a machine where I can test it, but the idea here is two conditions that check for the wp-content directory in the URI, and then a negated condition for the files you want to serve statically. Everything else should put out a 403 error. If it's not right, it will certainly point you in the right direction ;-)
edit: syntax should be %{} not {%} -- thanks Jeff!