a very good question. As far as I know it is not possible to create a usable 'press-and-hold'-macro. You can create a macro with the letter 'b' and delete the last entry (the key release), but this only leads to a mess (as if the keyboard 'b' key is constantly being pressed). There is always the possibility to edit the xml of the Mudbox profile and manually add another macro for the 'b' key release, and later assign this second macro to, say button 2. But I don't know exactly how to edit the xml for this purpose.
What works is to assign an axis to the letter 'b'. Tick the 'Press And Hold' radio button, and it should work. You may need to restart Mudbox.