Is it possible to turn on "read receipts" in iMessage for just certain contacts?
I don't believe this is possible at this time. Read receipts in iMessage appears to be an all-on or all-off option only.
This is core iOS functionality. Aside from looking for jailbreak-community mods (which may or may not exist) or writing your own modification (would require jailbreak) you're gonna have to wait for Apple to decide to include this feature (which they may never).
Individual read receipt assignment is coming in iOS 9, for what it's worth.