This didn't work, so I investigated a bit further. Because the = sign is included in org-link-escape-chars, the org-url-hexify-p value will not make anu difference.

So the problem is still that equals signs are hexified in org-links. Reading =org.el= and seeing the =org-link-escape-chars= constant this seems to be expected behavior.

So I think the issue is that the html exporter doesn't unhexify the equals sign in the link. As a result the link in the html doesn't work.

e.g.

The link: 

Is translated to:

In the org file

And that link is also included in the HTML where I would have expected the equals sign again.