When I draw an odd shape in silkscreen for a library part, I call it "copper", and hatch it with lines that are wide enough that it looks solid.
However, when that part is placed on a board layout, the hatching line width is reduced, so that now the "solid" areas are hatched.
In the library editor, there doesn't seem to be a "solid" option. And in layout, there doesn't seem to be a way to edit these silkscreen areas to change the line widths or make them solid.
I run into this when I make logos. I make them as a library component, so that I can place them in any design.