The group I am in creates all drawings from detailed fabrication drawings, to general arrangements and foundation details. It is very important to us to have different part of the model on different levels. It is not that much work to do it either. It's just a matter of selecting individual parts of the model an placing them on the correct level. Typically we design things in reference, so we also reuse our models many times within many different projects. They are separated into files though. Are you using file based or component based OpenPlant?
Yes, I am setting the transparency based on a .dgnlib predefined level. Since the graphics need to reside on a level anyway, I figured it was the only way to go. I first tried to set the transparency like you are showing, but I had the same issue. It did not work.