Ahoy CMDRs! Some Thoughts:
The Pleiades is an interesting star cluster - all the major stars in the cluster are moving together, and seem to have formed together, but the cloud of nebulosity they're moving through doesn't seem to be moving with them, OR related to their formation (from Wikipedia astronomy reading this past week). By this logic, some of the stars in the cluster are currently moving through the cloud, some have already finished moving through the cloud, and some have not/will not move through it. Given that the barnacle-entities prefer nebulae somehow, I'd be willing to bet that this could be used to narrow down the search a bit, on the system level.
I've also been paying attention to where and when the dusty-looking partial atmospheres of "volatiles" form - the fog you see on certain surfaces and in craters. So far, nothing over 900K seems to have this fog - Merope 2D, on the other hand, around 400-450K, has lots of it. I'm willing to bet there's a specific range of "fogginess" we're looking for, which is based on temperature and gravity.
Furthermore - and I don't have enough data on this one yet - I'm starting to suspect that on a number of worlds - Merope 2D in particular - the fog only seems to be thick on the daytime side of the planet. Combine that idea with the trailer footage... I think we might be looking for a greenish foggy crater, near the sunset terminus (sunset because it's in the already-heated part of the crust, where sunrise would be about-to-be-heated). I'm going to keep comparing temp, gravity, fogginess, and orbital period, and see which pieces of this hypothesis start to fall away.
If any of you have made observations that contradict what I've said, please share - because that's what science is all about!