I've been struggling with this same issue for quite some time, and finally stumbled over a solution (at least for me) in another forum. If all you need to do is give the APPEARANCE of a margin, try this:
Put your rich text control on top of a TPanel. Make the panel the same color as the rich text control, set the Rich Text control's align property to alClient, and set the borderstyle for both components to None. Set the border for the panel to whatever width you want the "margin" to be.
Hope this helps.
Mary