Poco,
The cause for the reciprocity failure lies in the formation of the latent image. When a photon hits a halogen ion, an electron is freed and floats thru the crystal structure until it finds a suitable disturbance. This negative charged disturbance attracts free-floating silver ions. When they meet, the silver ion becomes a silver atom. This forms the first nucleus of the latent image. However, a single silver atom surrounded by lots of silver ions has a lifetime of a fraction of a second, depending on temperature. It takes a cluster of four of these silver atoms to form a stable latent image that can be developed. The lower the light, the more silver atoms will convert back to silver ions before a stable structure can be built. This is causes the loss of film speed.

Although this might not answer your question directly, it may give you an impression of what will happen. Although bracketing might be a silly recommendation in your case, you will hardly be able to calculate an exact exposure.