Didn't BA already write off a 747 which according to answers to my queries, did not have a moving map airport display installed in an OPT which could have, and in my opinion, most likely would have enabled the crew to avoid leaving BA with a hull loss on their records.
depending on the method of compliance, and device used in any Portable EFB set-up, it's damned near impossible to get AMMD (Airport Moving Map Display) approved with the likes of a portable EFB, say using AMC 20-25 as method of compliance, and then satisfying the ETSO-C165a....needs at least an external aerial I would suggest for the accuracy needed for a decent idea of position without any "furyness" or dilution of aircraft actual position. Installed EFB might be tad easier, although guessing that isn't an option fitted for the older 747-400's??.