Quite a lot of the imported meshes I've tried under True Ambience do not render properly - either completely black or partly black.

Just a guess, but if the black 'speckles' correspond to individual polygons then there's a possibility of "reversed normals". I don't really know what a "reversed normal" is but I do know I've seen blacked out polygons in other renderers that have been attributed to reversed normals.

If Wings has a feature to correct reversed normals (I've seen similar on the export options of some modellers) then you could try that.

Bear in mind that your model will look facetted (non smooth) under TA anyway, so it might be easier to try another render method.