Lists of text is implemented exactly as you describe with
PluggableListMorph and its several progeny.

Lists of morphs seem to be hand-hacked with each special case.  You can
certainly make your own holder morph, and then have the holder morph
respond to drag and drop commands and query the underlying model.

Other than that, I dunno.  GUI code trips me up because I have trouble
drawing the lines.  Well, one thought: sometimes, especially in morphic,
you *shouldn't* draw the line, and you should let the view be the model,
too.

-Lex