Summary:
Basically, they hide the namecoin block hash in the coinbase transaction of the bitcoin block, and if a hash is found for the bitcoin header that meets the namecoin difficulty standard, the namecoin chain can use the entire bitcoin header to extend the namecoin chain, even if the bitcoin difficulty target was not met. It does not extend the header size, but it does extend the coinbase script field, which isn't a very big deal because it wasn't a fixed size to begin with.
OK, I suppose I will consider myself sold on it. This summary belongs in the Wiki, because I couldn't gather this from what was there.
Essentially what you are saying is that reference to the BTC block chain isn't ever needed, and that there is no dependency, but rather, that NMC will accept a "proof of work" formatted for Bitcoin as a Namecoin proof of work - and tack it onto the
Namecoin block chain - so long as that Bitcoin proof of work contains a reference to the most recent Namecoin block. And that NMC will look for the merkle root not in the merkle root field (since that's reserved for Bitcoin compatibility), but in the coinbase.
As a purely abstract argument, I assume this would mean one doesn't actually need to be mining real Bitcoins, the Bitcoin block chain will never be consulted by any non-mining Namecoin client, and that a block that ends up becoming stale or rejected off the Bitcoin block chain can still remain valid on Namecoins, and in fact never need to become part of the Bitcoin block chain, if they meet the Namecoin difficulty but not Bitcoin.
If I have understood this correctly, then I hereby do a 180 and endorse the idea and apologizing for not seeing the greatness (though the documentation could have been better).
In this case, why wait till block 24000? That could be a while, especially at the low end of this manic-depressive mining cycle. If the Namecoin exchange, the Namecoin block explorer, the Name DNS servers, and other critical infrastructure convert, the conversion can happen pretty much any time, because there will be no functional tools or any reason for anyone to stay on the old system.