Hi Mark,
A few tips...
The difficult part about brush passing on the Bonzini is getting the toss just right, especially with the imperfections of the play surface. Ideally, you will want the ball right under the rod (for cork balls), and a little bit further back but not quite behind the rod (for the ITSF "B" ball). If you have the all any further back, there is a good chance that you will stub during the pass. You may find that you can have the ball behind the rod with the "B" ball, depending on your brush stroke, but I would need to see you try a couple before I could say definitively. Due to the grippy nature of the ball/surface/man combination, when the ball is further back, you can execute a VERY steep brush pass, but having the ball that far back limits the ability to execute a shallower brush pass, thus you create a tell in the passing series. Also, the speed of the toss can vary from table to table, depending on the surface of the playfeild in the area you are tossing the ball. When I am brush passing, and the surface is not flat in the area I am passing, I tend to toss the ball a little faster so the ball is not as affected by the surface, where I would normally want the toss a little slower. As for whether you should start from the ball in a back pin, or start with the ball next to the players foot, (for me) that too depends on the surface, which ball I am using, and the table itself. Either way, practice getting the ball to the "sweetspot" for the actual pass. You will most likely see differences in the reaction of the ball during the toss when you change tables, or by simply trying the same pass on the other side of the table from where you normally practice.
Hope that helps a little, please feel free to ask more questions, if I didn't give the advice you were looking for.
Also, I am going to try to make it down for the SC tournament next month, if you are going to be there, I can show you some more stuff in person (ALOT easier to show than to write down exactly what I mean).
Pat