Looks like some compatability issues exist between Rewrites to the mod_cluster balancer and a ProxyPassReverse for it.

<VirtualHost *:80>
RewriteEngine On
RewriteRule ^/foo/(.*)$ balancer://mycluster/foo/$1 [P,L]
ProxyPassReverse /foo/ balancer://mycluster/foo/
</VirtualHost>

Requests served through the above VHost config fail with a 500. The log shows:

[Thu Aug 08 16:38:50 2013] [debug] mod_proxy_cluster.c(3448): proxy: CLUSTER no balancer for balancer://mycluster/foo/hi.jsp
[Thu Aug 08 16:38:50 2013] [debug] proxy_util.c(1525): [client 127.0.0.1] proxy: *: found reverse proxy worker for balancer://mycluster/foo/hi.jsp
[Thu Aug 08 16:38:50 2013] [debug] mod_proxy.c(1015): Running scheme balancer handler (attempt 0)
[Thu Aug 08 16:38:50 2013] [debug] mod_proxy_http.c(1955): proxy: HTTP: declining URL balancer://mycluster/foo/hi.jsp
[Thu Aug 08 16:38:50 2013] [debug] mod_proxy_ajp.c(664): proxy: AJP: declining URL balancer://mycluster/foo/hi.jsp
[Thu Aug 08 16:38:50 2013] [warn] proxy: No protocol handler was valid for the URL /foo/hi.jsp. If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.

Removing the ProxyPassReverse, the request works again. Looks like the balancer isn't found for some reason with both the Rewrite and ProxyPassReverse.