That sounds really awesome. Do you have a screeny you could post to show the result?
And yeah... I know it is a pain to remove the trees around the edge of my forest sheet, made worse by the fact that the style I used had them all pretty damn close to each other. But I've used it a few times now and it is a LOT faster than doing the whole forest by hand a brush at a time. But yes, it is fiddly.
This sounds like it could be really cool.