Spurred by all this I sat down and added DNT support in Epiphany, which thankfully is an extremely simple spec to implement. It’s now in master, so anyone willing to enable just needs to go to the Privacy tab in Preferences and click:
Now the pages that choose to respect this setting (unfortunately not everyone does; by a long shot), should be able to detect your request. We can see that things are working in the donottrack.us page itself.
Note that the page claims our browser does not support the feature, yet it is enabled; this is because DNT being an HTTP header extension the only way for the page to tell you whether you browser supports it in theory is by having a hardcoded list of supported browsers, which does not include Epiphany. Oh well. Either way, enjoy your newly untraceable goodness, which should make its way into the next unstable release.