Hi David,
I am not really answering your question, but I rarely use the brush tool and default to the paths tool most of the time to make a mask that in Lightroom I would have made with the brush tool. If you have already used and appreciate the power of the path tool for mask in DT then just ignore this comment and I apologise for the noise in this thread. If you have never really explored the path tool to draw a mask it is a very powerful way of working.
I do agree David that the brush would be really more useful as a tool if you could see the effect happening instantly and also if the brush could be turnedd into an eraser by using a keyboard shortcut so you could rub out mistakes. But that is a Lightroom approach and it would mean creating a new masking tool and I presume would be a huge amount of work. That is why I prefer the path tool as I can manipulate the path tool after it is drawn if I make mistakes. You can of course still manipulate the brush tools mask as well. So use which ever tool you prefer to use.
Good luck.