I think this'll complicate axis placement, but if this can be done without negatively affecting these at various radii, I'm not against this. However, I think this would be best parameterised as a margin/padding/spacing, rather than giving direct control over the radii.

That makes sense. If we parameterize this as a margin/padding/spacing, would it conflict with the plot margin defined in the theme? Or would it be appropriate to make this parameter an element within the theme itself?

I don't think the theme is available when data is being transformed, so I don't think a theme setting would make a lot of sense from a plumbing perspective. There'd be a slight semantic overlap between the spacing and the plot.margin, but I'm not too worried about this.