I have always assumed that if you took the rules for making a 1st level multi-classed character using two 0 level 'apprentice' versions of a class, and instead just had a single 'apprentice' class that it would approximate a 0 level character quite well.
Regressing one level back makes all characters basically 1st level commoners, and first level commoner exists as a class basically to approximate AD&D 0 level characters.