I think that the GL class draws in the same moment you call one of its draw functions. As the Queue tag would define when an object is rendered, it is just ignored. I am not exactly sure if I understand your problem correctly, but as you already draw after everything else, by doing it within that OnPostRender event, I guess that it has to do with depth testing. Transparent objects usually dont write into the depth buffer and you also disable writing into it for your custom drawing. This means that if you want it to be behind transparent objects, you have to draw it before drawing anything transparent, or if you want it to be drawn in front of solid objects, you have to set the vertex position correctly or disable the depth test.
I hope this makes sense...
