This is a sample of how Heaps can be used to take a screenshot (at least of what is inside of the Heaps app window):
var tex = new h3d.mat.Texture(640, 480, [Target, IsNPOT, Serialize] );
s2d.drawTo(tex);
var pixels = tex.capturePixels();
sys.io.File.saveBytes( "my_screenshot.png", pixels.toPNG() );
(Thanks to Rudy helping me figure this out )