In the bottom of the page just simply do not cling to the module HttpAdditionModule connected. I tried to specify any page as proxy_pass, nothing connects.
With add_before_body slightly different story, it simply replaces the page to the one shown in proxy_pass.
What does he want? Help please ...

location = /after_action {
proxy_pass http://127.0.0.1:81/help.html;
proxy_redirect off;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
location / {
...
add_after_body /after_action;
...
}

version nginx 1.0.5