Is it possible to add or subtract a correction factor to the z axis zero to accommodate an engraver with a curved bed? E.g. a look up table for various x,y positions with linear interpolation between points.

Ther is a way to do it but it is not easy and can cause all sorts of issues. There was a discussion a while back about a similar thing on the Yahoo group and even Art recommended it not be tried.
 If possible it would be a lot better to put some sort of sacraficial plate on the top and shim it or better stil if  your engraver was capable of revolving a flycutter then surfacing it.

Hood