I am trying to configure an SSH connection through a proxy server such that

Host bridge-test
  HostName TARGETHOST
  ProxyCommand ssh -W BRIDGEHOST %h:%p

however, my problem is that I do not know a priori what TARGETHOST is. This information can only be accessed by executing a command on the BRIDGEHOST (which is static). Is there any way to configure SSH to pick up the TARGETHOST as a variable from the proxy server?