On the getpaint.net website it was stated that the forthcoming version will include antialised selections. Based on what antialising is on the "Clone Stamp" tool seems to me like it is a one pixel feathering. I would like suggest you add a built in feathering option (with a user defineable pixel width) for all the tools that it is aplicable to such as the selection tool, clone stamp tool and eraser.
I was pleased to find out there is a plugin (or effect as you call it) called "Feather selection...". The plugin seems to work with the selection tool but not with the clone stamp tool.
