Costum 503 error page for apache with mod_proxy

I recently needed to create a custom 503 error page for an apache that serves multiple vhosts via ProxyPass.

Consider the following situation:
We have an apache proxy that serves a bunch of internal webservices via mod_proxy.
These webservices all have nice costum error pages esp. for the classical 404 - page not found.
Every now and then one of these Servers is not reachable, due to maintenance or heavy load. If this happens, the apache running mod_proxy will show an ugly standard 503 error message.