Hiya,
I did check the EffectEnvironmentParameters and couldn't see much. It seems to provide BackColor, ForeColor and BrushWidth along with a GetSelection method which seemed promising but takes a Rectangle and returns a PdnRegion which doesn't seem quite what I wanted. Although I haven't worked long with this so it could be I'm misunderstanding the method
I thought maybe the rectangle could be a subset of the image, allowing you to grab a part of the selection only. But if I pass in dstArgs.Bounds I simply get the surface bounds again.