That's a funny thought, but the company making the complaint could probably prove that the ad blocker was interfering with their ads even without looking at the source code.

Moreover, the DMCA covers unauthorised access to copyrighted content, and the ad blocker cannot claim ownership of the ROT13'ed domain name, just as the domain name itself is not copyrightable.

In theory the ad blocker could use a more complicated scheme to obfuscate their source code, but I'm not sure whether they could combine a "do not de-obfuscate this code" rule with an open source / Free Software license.

This is a crucial point: the software is not blocking their ads, neither are the software's authors--the users of the software are. And the users have every right to not connect their computers to any other computer they please.

It's interesting to compare this to Second Amendment arguments. Do ad blockers block ads, or do users? Do users have a right to keep and bear ad blockers? Of course it's silly, and ad blockers are passive tools, but there are some striking parallels.