HIT as Assistance Work?

I am doing CW’s 10x3 for fat loss, and was using CT’s idea of 15 min at the end of the workout to do whatever - mostly beach body stuff. I am experimenting with HIT principles during the assitance work.

My question is would the fatigue brought from going to failure be counter productive to whatever muscle stimulus was created, considering I am on a fat loss phase and hence hypocaloric, doing some cardio, etc.

I believe CS did something like this during his Velocity Diet experiment.