Another long shot is to take it into the cloth room and clothify the hair.  I've had it work for some older prop hair that had no movement morphs but the results were inconsistent.

Mitsu or Sylphiad hair has a morph for lying down.  I can't check as I'm not at my home system.

Pretty3D's Fantasy Warrior Hair has internal bones that allow for some extreme posing.

Kaposer sell TotalHairCntrl here in the marketplace.  It's a magnet based system that can be used on any hair.  It mainly designed for prop hair that lacks movement morphs. 

You can also try applying your own magnets if you don't want to spend any more money.