There are a few essential elements for the proxy_mod to set. We have done a few experiments with it and we collected settings which seem to be working fine with JWChat client (JsJac library) and our xmpp4gwt library which is used in our minichat and other clients.

Here is my proxy_mod configuration for minichat client:
(I should probably write a guide for this.)

<VirtualHost *:80>
  # Usual stuff with virtual domain configuration:
  ServerName minichat.tigase.org
  DocumentRoot /home/webapp/minichat
  ServerAdmin admin-accATtigase.org
  ErrorLog /var/log/apache2/minichat.tigase.org-error_log
  CustomLog /var/log/apache2/minichat.tigase.org-access_log vhost
  # You might want to do automatic redirect from simple address
  # to the correct HTML page with JavaScript code
  # Not used in our env right now
#  RedirectMatch permanent /$ http://minichat.tigase.org/App.html?jid=@tigase.org
  # I think this is important for Tigase Bosh component which is NOT
  # HTTP-1.1 compliant server, it just pretends to be HTTP server and
  # has a vey basic HTTP implementation
  SetEnv force-proxy-request-1.0 1
  SetEnv proxy-nokeepalive 1
  # Setting for a directory with the client code and files
  <Directory /home/webapp/minichat>
    Options  +Indexes +Multiviews
    AllowOverride All
    Order allow,deny
    Allow from all
    # This section is important for GWT based applications
    # Switch web browser and proxy caching for selected files.
    <FilesMatch "\.nocache\.">
      FileETag None
      Header unset ETag
      Header set Cache-Control "max-age=0, no-cache, no-store, private"
      Header set Pragma "no-cache"
      Header set Expires "0"
    </FilesMatch>
  </Directory>
  # This is critical for Bosh to work properly - set UTF-8 charset
  AddDefaultCharset UTF-8
  # Standard proxy_mod stuff for requests forwarding.
  ProxyTimeout 600
  RewriteEngine on
  RewriteRule /bosh http://127.0.0.1:5280/xmpp-bosh/ [P]
</VirtualHost>