Hi Alex,
Late to the references party!
I agree with Doitsu. Indeed, according to Littré the term means (mot à mot): "cover in feathers". So when he is "pushing back the plumeau" he is pushing back a feathers blanket.
Ref:
http://www.littre.org/definition/plumeau See def. # 5. The Littré is often a good French source for old words.