What is the recommended way to setup a web proxy to another server on the LAN?

I'm in no way an expert on this, but I think you can do what you are asking using the WebApp feature.

You make a plist in webapps folder and a conf file in the apache folder and then using Server.app, edit your website and click on the Edit Advanced Settings button and tick the webapp that you created. This shouldn't then get over written if you make any changes.

The conf file contents would have the proxy information in it:

# from http://httpd.apache.org/docs/2.2/mod/mod_proxy.html#proxypass

RewriteEngine On

RewriteCond %{HTTPS} =off

RewriteRule . - [E=protocol:http,E=port:8000]

RewriteCond %{HTTPS} =on

RewriteRule . - [E=protocol:https,E=port:8010]

ProxyPassReverse / http://secondserver.example.com:8000/

ProxyPass / http://secondserver.example.com:8000/

and the plist would be something like this:

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">

<!-- See man pages for webapp.plist(5) and webappctl(8) for information about this example webapp.plist -->

<plist version="1.0">

<dict>

<key>includeFiles</key>

<array> <!-- Include files are activated in virtual host when webapp is started -->

<string>/Library/Server/Web/Config/apache2/httpd_yourwebapp.conf</string> <!-- Name should match your conf file-->

</array>

<key>name</key>

<string>com.example.secondserverwebapp</string>

<key>displayName</key> <!-- Name shown in Server app -->

<string>SecondServerWebApp</string>

<key>installationIndicatorFilePath</key> <!-- The presence of this file indicates web app is installed -->

<string>/Library/Server/Web/Config/apache2/httpd_yourwebapp.conf</string>

<key>sslPolicy</key> <!-- Determines webapp SSL behavior -->

<integer>0</integer> <!-- 0: default, UseSSLWhenEnabled -->

<!-- 1: UseSSLAlways -->

<!-- 2: UseSSLOnlyWhenCertificateIsTrustable -->

<!-- 3: UseSSLNever -->

<!-- 4: UseSSLAndNonSSL -->

</dict>

</plist>

This may not be completely right but may point you in the right direction, hopefully someone with more knowledge will chip in in the meantime