Experiment brush is currently limited to sharp edged results (either with or without antialiasing)
Providing a feathering option would allow it to be used also for effectively constructing a 'base image' for further airbrush-style work. Using Free select (with feathering) in GIMP followed by filling with FG color gives an accurate picture of the visual effect obtained. Among other things, it allows the quick and dirty construction of ad-hoc, arbitrarily shaped gradients.

I'm looking into implementing this. Any pointers at where best to begin integrating the existing gaussian-blurring (or box blurring ?) code with the plain rasterizing the Experiment brush currently does  would be appreciated.

I write this in somewhat of a hurry, but it seems to me that more than simply two options "[] feather"
and "Feather radius" are appropriate. From my experiments mocking the workflow up with GIMP, it seems that feather radius frequently needs to scale according to the size of the shape drawn. I am therefore considering that an additional option "Feather scaling" may be useful (a float value that may be set negatively, so that smaller areas are more feathered, or positively -- the usual --, so that larger areas are more feathered)

Reproducible: Always