I can’t help with the 787 question.
But it is interesting to note that our B737 Company iPad EFB’s have no problem getting GPS position when mounted in the bracket on the lower sill of the side window. It is very useful for ownship position on the Jepp App - particularly on the airport moving map during taxi. I guess the 787 has more shielding.
The iPad tends to lose GPS if you remove it from the bracket and place it on your lap.
Also, our Company is in the process of installing a feed from the Aircraft GPS (or perhaps FMC) position to the iPad for the same purpose. I think that connection will be via a USB charge port cable, although it could be Bluetooth - not sure. I haven’t seen one in service yet.