Apparently it's possible to communicate with and eventually ally with the Dread Lords in Dark Avatar (v1.50.059).

I was recently playing a game where I was at war with the Korath after a "Dread Lords Return" mega-event. Eventually I get a popup letting me know that the DLs want to help me in the war. I can, if I want, click "speak to" and trade with them. This eventually leads to being able to ally with them (as the popup returns every time they send me a ship or whatever). However, you can't break the alliance because they don't appear in the race lists.

While fun, Kyro tells me I shouldn't be able to communicate with the DLs at all, so here I am. If a savegame just before they contact me would be helpful, I can provide one.