Set up an HTTP proxy to insert a header

I do something like this in my development environment by configuring Apache on port 80 as a proxy for my application server on port 8080, with the following Apache config:

NameVirtualHost *
<VirtualHost *>
   <Proxy http://127.0.0.1:8080/*>
      Allow from all
   </Proxy>
   <LocationMatch "/myapp">
      ProxyPass http://127.0.0.1:8080/myapp
      ProxyPassReverse http://127.0.0.1:8080/myapp
      Header add myheader "myvalue"
      RequestHeader set myheader "myvalue"   
   </LocationMatch>
</VirtualHost>

See LocationMatch and RequestHeader documentation.

This adds the header myheader: myvalue to requests going to the application server.