I agree with most of whats been said in this thread. It is virtually impossible to drive a rhythm on the bass end of a semitone box, i.e. Bc cc# C#d etc. The bass can be used to ornament the odd treble note to great effect. A great deal of 'bass rhythm' can be generated by techniques such as pulsing the bellows and playing in a stacato fashion - fidlers etc manage it without a separate bass section.

The simple answer for those who prefer the chromaticness of a semitone box but want decent bass accompaniment is to move onto a 3 row BCC# British Chromatic system with accordion bass (as played by Jimmy shand, John Kirkpatrick etc etc) the right hand fingering is easier than on the 2 row box and a greater range of right hand chords can be played.
There may be one or two 2 row Bc boxes around with 12 accordion bass - hohner did a 12 accordion bass double ray delux but I havnt seen one for a long time.

I am in the process of writing a comprehensive tutor book covering both the 2 row BC and also dealing comprehensively with the 3 row BCC#.
I would be grateful for any commentys/ideas etc as to what to put in and what to leave out so that the book really does meat the needs of players and would be players.