This is actually a couple days old but was just brought to my attention. Valve has implemented initial support for Optimus!

http://psteamcommunity.yuanyoumao.com/app/221410/discussions/6/846939854395110175/

Notes:

• Make sure to upgrade to the latest build of Primus or you will experience crashes. The overlay also works.
  
• The instructions mention modifying '/etc/environment'. This is not required. To use Optirun or Primus on a game, simply modify the launch options to:
  primusrun %command%
  or
  optirun %command%