Ditch the proxycommand and just set the ssh config to connect to zbmc.eu directly, and set the hostname for zbmc.eu internally with dnsmasq on your internal lan via /etc/hosts as already suggested. I don't see why you feel this won't work.
Failing that put a script into ifup.d to detect which network you're on and write a line to /etc/hosts or comment it as appropriate (if you really must).
Adam