I've unsuccessfully searched Google and this group for info regarding filling a shape with a texture. I already tried filling with texture, but OpenToonz replaces the desired texture with a solid color of the average of the texture. This image shows the approximate result I'm going for. I was considering drawing what I want in GIMP and importing it, but that sounds like overkill, right?
Something else to keep in mind is that textures only apply to vector levels. Using a texture fill on a toonz raster level also has the result you describe.