Does anybody know whether there is an impending LR 5.6 Release Candidate that will support the Nikon D810?
Of course there is! There's
always a pending upgrade to support new cameras.
As for when: those that know won't say, and those that say don't know.
However, on past performance LR5.6 will be released at the same time as ACR 8.6, and we already have the RC.
If it's any clue (and who knows?) the RC for ACR 8.5 was released on 3rd June and the release versions of ACR 8.5 and LR5.5 were released on 18th June.