Namecoin - How to Find Block Height via Block Hash?
At the time of asking, this functionality wasn't supported in Namecoin. I've written a patch that adds this and a few other properties to Namecoin's getblock RPC call.

https://github.com/namecoin/namecoin/issues/87
https://gist.github.com/ryancdotorg/5e610e9ee546beed367d