So it's been about 2 years since I've posted on these forums, I still browse from time to time to see what good stuff PMDG has cooking.
Anyway, here is an interview from 2013 in which Robert Randazzo is asked this question:
For those who can't view the video here is the transcript:
FroogleSim: "Are you considering a 787?"
Robert: "Right now surprisingly we are not. We haven't gotten the data package yet these things cost money. There's a couple of other things we want to do first but I'm sure we will get there some day".
FroogleSim: "Isn't a 787 just a very big 777 but you turn it on and it catches fire?"
Robert: "....the 787 is a marvel of technology, from a developers stand point it just makes me break out in a very prickly rash because it's just so software driven my fear is there is just some whole stuff in there which we would have to invent just to support the data drive side of the aeroplane. But we will get there some day I'm sure".
So there you have it, there will probably be a PMDG 787 one day but it would be so hard to develop I'm not sure if it will happen any time soon.