As far as I know it is not possible to use any Opencockpits hardware with the old PMDG B747 - at least in FSX, because the addon does not use standard offsets and no SDK has been released.
For the upcoming PMDG B747 v2 an SDK will be released and I think the driver for the OC-Modules will be released shortly thereafter, as it was with the PMDG B777.