Thanks to Spiney for this tip. Baking an AO map in Blender and multiplying it onto the texture in Gimp (after some airbrushing in a layer mask, to make it look less blocky) does improve the look of the mapmodels.
With ambient occlusion baked and mogrified into the diffuse map:
Without ambient occlusion (notice the inside of the urns):
So yeah, that one’s a keeper.
Baking an AO map in Blender is very easy, and layering it onto the diffuse map in Gimp is also easy. It just improves the shadowing on the models; the urns are now shadowed on the inside and at the bottom as they should be. It is basically a hack, as in “painting shadows on the model”, but it works in this case.