not proper by any means, never supposed to rappel on a rigging line
that being said, I quite often set a rope to pull a stem or whole tree over, and rappel down the rigging line on a figure 8, again, NOT ADVISABLE, NOT SAFE, NOT ANSI APPROVED
just how I do it, rescue 8 descender, prusik backup on it, works great, as does a munter hitch but good luck doing a munter with such a large rope