I must say I don't really see the point of doing soft edge in video mapping… Soft edge is useful on a flat surface when you want to have a seamless image using multiple projectors. In a mapping context, you can always manage to have projectors intersection in a corner or some other physical element. And using the madmapper surfaces, you can very easily and very precisely align your image.
I did a 180° projection using 4 very distorted projectors using madmapper and it went very well except the fact that some projectors were more distant than others and it affected the brightness of the image but even with soft edge it wouldn't have help.
See the result here:
http://lumens8.ch/web/realisations/79/detail/717