Totally safe for the amp, within reason. I say within reason because although the act of attenuating (by way of FX loop or post transformer) is not harmful to the amp, the human tendency to now crank the master volume to full or close to it, can be. Even then, worst thing that can happen is lowered power tube life, or bad tone. Another thing to think about is that when using a floor pedal which was designed for passive guitar signal on its input, likely has an extremely high input impedance, and ideally, the loop wants to see an primary impedance lower than typical instrument impedance by a factor of 10 or more. It may overload the input section of the pedal. Overload in this case doesn't mean you have an exploded pedal, but likely the input of the pedal or the output of the FX send circuit in the amp, can run hot and distort, the result would be poor tone. Using a piece of line level gear specifically designed for -10dB to +4dB level signal and using a relatively low impedance trimmer would be much better, assuming that you want the cleanest attenuation and least coloring.
