I wonder if (in theory) it wouldn't make more sense to address this sort of thing at the system resolver level rather than in the browser, the way tor does.

Deeper OS integration would be nice, but harder to roll out. I think the plan early on was to make something useful/generic/agnostic enough that a couple of big browsers would add support, then it could try becoming a Web standard.

Unfortunately that hasn't panned out; but it still feels more feasible than getting Microsoft and Apple to add it to their OS network stacks.