Use mod_cluster with a JBoss with the application myapp (myapp.war) and use the following in httpd.conf:
RewriteEngine On
RewriteCond %

^cluster\.domain\.com [NC]
RewriteRule ^/$ /myapp/MyCount [PT]

As /myapp is mapped to JBoss you would except / on cluster.domain.com to go to /myapp/MyCount in JBoss unfortunately it goes to /.
You would except it goes to /myapp/MyCount as it does when to you a ProxyPass directive:
ProxyPass /myapp http://localhost:8080/myapp

Internals:
[debug] mod_proxy_cluster.c(1703): proxy_cluster_trans for 0 passthrough:/myapp/MyCount (null) uri: /myapp/MyCount args: (null) unparsed_uri: /
in get_balancer_by_node() we use r->uri (/myapp/MyCount in the case) to map to the application.
in proxy_cluster_trans we do: r->filename = apr_pstrcat(r->pool, "proxy:balancer://", balancer, r->unparsed_uri, NULL);