I am quite sure Jeppesen/Bottlang does not do any calculation regarding airfield elevation. Normally (always?) they re-publish data off the National AIP which defines which height datum is used. Normally this will be the nationally defined MSL.
As GPS is not (yet?) in use for height measurements in aviation, the issue is not operationally relevant.
If you want this confirmed, why not ask Jeppesen? You can access their homepage on
www.jeppesen.com.
Is this satisfactory?
LIN