Is jet fuel typically injected before or after the flame holder in an afterburner?
This is what I could find on the matter in my university textbook. 


  The fuel spray bar is typically composed of
  one or more rings with distinct fuel injection heads circumferentially distributed around
  the ring. The V-shaped flame holder ring(s) create a fuel–air mixture recirculation region
  in its turbulent wake, which allows for a stable flame front to be established.




Although it does not mention fuel heating explicitly, I would agree with your assumption -- as the mixing of the hot gas and the (colder) fuel of course also evens out temperature differences between the two.

Source: Farokhi (2014), Aircraft Propulsion (p. 199)