Have you seen DirectDrawGraphicsMBS class in latest MBS Xojo Plugins pre-release 3?
It allows you to draw lines, rectangles, ellipses and round rectangles with dashed lines and transparent colors.
All configurable as needed with standard or custom dash patterns. You can also set line width, what dash caps to use and what line joins to use.
We may add more methods if needed. Currently we wrap ID2D1RenderTarget interface in DirectDraw and provide a couple of methods to access feature there.
For MacOS you can use CGContextMBS class.