HTTP request with SLA

Here's a (nonAI) problem I'd like to solve. Configure apache to receive a request and proxy/forward it off to a backend app server (tomcat) .. wait a specified period of time ... if no response is received send back a static file or a return code like 204.

Essentially it's a combination between the below mod_rewrite and mod_alias directives with a timeout. Below example uses solr without loss of generality.

RedirectMatch 204 /search/(.*)$
RewriteRule ^/search/(.*)$ http://backend:8080/solr/$1 [P]

Logic below:

If url matches rewrite-rule regex then
{
        set timer for 500ms with timer_callback()
        force proxy to second url after rewriting it
        if(response from proxy is > 199 and < 300)
           return response
        else
           return 204 or static default file
}

timer_callback()
{
       return 204 or static default file
}

Instead of returning 204 one could also serve back a static file like /noresults.xml

The general idea is to expose a url that has a near-guaranteed response time limit (assuming apache is alive) where a 204 or a static default is acceptable behaviour. I suspect that we'll need to write an apache module to do this, yet surely this question has been asked and solved before!