I forgot about this thread. I suppose I should update it.
Shortly after writing this thread we did give Modern Bill a try. We spent many hours on it configuring it and then gave up. It is many orders of magnitude more complex than Client Exec and really unintuitive.
Now that CE 3.0 is on the horizon, things are really looking up. We're running the
PR 2 of CE 3.0 and it's quite nice.
I withdraw my recommendation for Modern Bill.