You could overwrite ZO_WorldMapContainer1.GetTextureFileDimensions:
Code:
local GetTextureFileDimensions = ZO_WorldMapContainer1.GetTextureFileDimensions
local myOverZoom = 2
ZO_WorldMapContainer1.GetTextureFileDimensions = function(self)
return GetTextureFileDimensions(self) * myOverZoom
end
Tried it and it seems to work for me.