I've watched this for any suggestions but nobody seems to be taking it.
If I understand you correctly, I'd urge you not to try messing around with the text view's drawing routines, especially when the Cocoa text system has so many hooks at so many levels?
This is essentially a paragraph-level formatting problem, isn't it? Why not just set the paragraph's NSParagraphStyle? You can do this by setting its headIndent, which is the margin ahead of the beginning of the user's language's sweep path (left-to-right, top-to-bottom, etc.).
You can intercept -insertTab: and -insertBacktab: using NSTextView's -textView:doCommandBySelector: delegate method and take that as your cue to modify the paragraph style to indent as far as you want.