Thanks stwert. Experimented a bit and the results are not quite what I expected.
As you suggested I used a cube, reversed the normals, camera inside, default lighting off, AO shadow color set to black, AO light color set to white.
First image is a 400mm cube with ao_distance of 400. Almost black as expected with values less than 400 brightening up the scene fairly quickly.
Second image is a 400cm cube with ao_distance still at 400.