Quick addition to the last answer: If you use "getrawtransaction" to extract a tx with Namecoin Core, you see both the name and the value in the JSON. In this way, you can prove that your hash is in the tx value very easily with just a single RPC call (all you need to know is the txid).
If you want to go fancy, you could also try to use my Namestamp tool (see the blog post I linked at the beginning of this thread). This allows you to create proofs linking your hash value to a Bitcoin block header. They are self-contained, so you don't even need the Namecoin blockchain, txid or name to show that your hash existed before a certain Bitcoin block.