The ComboBox gets its RightToLeft property from the GoView, if GoView.RightToLeftFromView is true, or else from the GoText.RightToLeft property.
Well, I just posted an example of customizing GoText to create customized ComboBoxes: http://www.nwoods.com/forum/forum_posts.asp?TID=2474.
I suggest you start by trying the simpler case, overriding DoBeginEdit, to see if that works. If trying to set ComboBox properties after it has been created fails, then you’ll need to pursue the course presented by all of that code, overriding CreateEditor and including the ComboBox-inheriting control.
You probably won’t need an override of DoEdit (or use of DoEditEx) which was the other reason for that sample code.