I think I mentioned it recently as a good backup option. I think there are probably two primary reasons why it hasn't been discussed much yet.
First is the unknown. With no contacts, it's too hard to predict whether a diplomatic victory is available. To win diplomatically, you really need a coalition of several mutually friendly civilizations. If everyone already hates each other, that's probably not going to be possible.
Second, regardless of whether a diplomatic win turns out to be possible, there's not much we can do about it now. Whether we are or aren't working toward a diplomatic win, the short term strategy should be more or less the same. Once we start meeting people, then we'll need to be careful about our dealings with them, but for now it really doesn't matter. There are really only two things I can think of that we can do now that might affect our chances for a diplomatic victory.
We could aggressively research toward Philosophy and Divine Right (and Theology? Has Christianity been founded?) in order to reduce the amount of religious diversity in the world. That would improve our chances of finding a bloc of allies that we can court as our diplomatic coalition. It might already be too late for that anyway, with Hinduism, Buddhism, and Judaism already loose in the field, but it's an idea. Either way, I think that path has already been suggested since it plays well into a cultural victory, so I don't see that as a strictly diplomatic strategy.
We could also make a case that the Optics / Astronomy beeline would be beneficial for diplomacy. That would get us earlier contact which would give us more time to develop good relations. That's a valid point, but I think you could just as easily argue that the other way. The earlier you meet everyone, the more time you have for them to hate you for your religious differences before you switch to Free Religion and/or to suck you into unwanted wars. I think you could go either way on this one. I'd be inclined to ignore this issue, especially since I'm of the opinion that the Optics beeline isn't really beneficial to our other options.
I'm a big fan of the diplomatic victory, and I think Sisiutil has only done it once in the ALC's and that only after a lot of prodding

. This might be an opportunity to notch diplomatic victory #2, but it's way too early to tell or to do anything about it.