Namecoin and namecoin-derived systems are cool, but they tend to overlook a lot of real-world functionality provided by DNS. As used today DNS is 1) A distributed key-value storage system 2) Incredibly scalable and fault-tolerant and 3) a de-facto internet routing layer for CDNs and GSLB endpoints. Namecoin systems typically provide (1) but overlook (2) and (3), which makes it hard to consider them as viable replacements.
I think it's interesting to look at what existing entities do when faced with DNS MITM and takedowns. The various torrent searchers and anti-censorship entities just diversified the TLDs they depend upon. So when their ".com" or ".net" domain gets taken down or man-in-the-middled, they tell their users to shift to .is , .ch, .se or some other TLD with a different regulatory framework, thus avoiding a single point of failure.
If a new mechanism depends on the inconvenience of a browser extension anyway, why not automate the process people already use? For example "colmmacc.multi" could be intercepted by an extension and translated into 5 DNS requests against say SHA-2("colmmacc").[com|ch|ly|se|is] and the extension could use a simple majority quorum of the answers to defend against a tampered response. Of course it means you have to register and host your domain 5 times, but that's pretty cheap these days.
Other nice properties: works with all existing DNS security mechanisms (including DNSSEC or DNScurve), provides security against registrar or registry level tampering or compromises. Hash of the domain makes it hard for registries to block domains (they have no idea what the name is until it is popular) and also resets the clock on squatters.