Apache2 mod_proxy without 301 moved permanently

I'm not sure this'll work, but you could try it:

<ProxyMatch http://[^/]+/.*mysign>
    Allow from All
    Order Allow,Deny
    RewriteEngine on
    RewriteRule . http://localhost/mysign/ [P]
</ProxyMatch>

If the http://localhost/mysign/ URL is being served by the same server which is acting as the proxy, then you could try replacing the RewriteRule line with this:

    RewriteRule ^.*$ /mysign

There is way, but not using 404, and to be honest it's a bad idea.

First, set maxattempts to a number bigger than 1. Set nofailover to Off.

Then in the backend that does not hold the object you want, you return a 503 when you want it to try the next one. (503 = HTTP_SERVICE_UNAVAILABLE)

This won't work in the real life though. Apache has no way of guessing what's a real 503 error, and what's a "try the other server" error. In other words, it will break the real 404 detection. There will be no way of issuing a 404 "page not found" error, since you'll always will return "service unavailable" errors to the users after apache gives up trying all servers.

Besides, the thing you're trying to do here is not a good idea, IMHO. You should configure proper mappings on your Apache Proxy, and assign servers based on the URLs of your requests.

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