Measuring loop Gain and phase of of a flyback converter
with points you get more edit privies.... I honestly am not sure the best way to Bode Plot this non-linear current-mode dual feedback path switching regulator in an analog open loop voltage gain transfer function.  So I think the best way is to measure the current loop on secondary. Injecting a sinusoidal current over a DC load current and measure the voltage response on the output. With the assumption of injecting a linear sweep f error voltage I would use a biased MOSFET and a resistive load in shunt. 


  I realize this is more of a z22 plot than a s21 plot. 


But there is no single feedback point and FB is PAM modulated goes inside to a S&amp;H  and mixed with tertiary zero-crossing current sense to a set-reset flip-flop to control PWM.

So try this and consider a 1 Ohm active load sense with a pot to change the operating DC load and add various levels of AC load from 10% full scale to 25% to 50% with a 10% static DC load to improve stability.  Then plot the z22 response whereas the inverse y22 is the load regulation error as a function of frequency using your DSO &amp; excel.  The Audacity log sweep is useful here.