If anyone wants to contribute this, I'm sure it would be welcomed. I'd
recommend having an option that sets some area threshold so the user can
specify "remove islands less than X mil^2 in area".
I think the challenge is going to be that the copper fill is implemented
as a polygon with clearances drawn on top of it so island removal will
entail drawing more clears. This is as opposed to a fill which is done
with lots of line segments where you simply delete those which don't
connect anywhere.
It may be useful to look into the ability to specify that a copper fill
is done with lines vs a polygon, but I haven't really put much thought
into that.