Doesn’t look like this is easily do-able, not all the display mode parameters are exposed in RhinoCommon. A possible hack may be to change the object color setting - one of those settings which is exposed - to something that has a lower alpha channel value. But I’m not sure that will cover all situations.
Edit: actually, that doesn’t work, I can change the object color alpha value, but it doesn’t affect the display. So, in the end, I don’t see any controls on Ghosted transparency exposed to scripting.
Macro-wise, the only current possibility is to have set up a copy of the display mode as Randy suggested and switch to that with a macro.
If you don’t need to change things while a script is running, then the best bet is probably just via the Display panel.
Edit#2 One other thing that might be possible is to set the object(s) in rendered mode via SetObjectDisplayMode), then change the transparency of the render material…