<penciling in...>
I have two prior commits for the weekend (CCC and Elven's burning body), but I'll work on a little tutorial if I get the time.
Short version: Color the vertexes shades of grey (black is loosey goosey, white is stiff as a board - and yeah, only the red channel is supposed to count, but just do the shades of grey). Add an Aurora flexmesh modifier (winging the name as I can't fire up tools right now) and adjust the parameters.
So make the helm part all white vertexes (stiff) and paint the feather vertexes increasingly darker shades (with the tip ending up around 128 or so - more would be too wild)
<...a quick sketch or two>