Buffering the input to a passive RC integrator used to measure magnetic flux density
I'm also wondering, since I'm using a passive integrator, does the EMF
  induced in the measurement coil need to be buffered before being fed
  into the integrator?


In short, yes because it eliminates the need to account for the losses in the resistor.

Without having to invoke circuit theory for a lossy secondary, to measure B we need to measure V as accurately as possible, which means open loop. 

$$B = \frac{\Phi}{A}$$ and $$ V_s = \frac{d\Phi}{dt}*N_s $$

if you draw current from the loop then you need to account for the energy loss, which would mean factoring in a current into these equations because the voltage drop on the secondary would need to be accounted for. Much easier to use a high impedance measurement on the secondary.

In all of the hysteresis measuring curve circuits I have seen only high impedance integrator (which implies active).