Paralleling a slow ramp and toggling pulse current
sources is similar to FvM's suggestion but perhaps a
bit easier to create. It will not give you canonical
Bode type results but let you eyeball large signal
overshoot (which I view as more useful a challenge).
However, even here you will find that behavior is
going to change depending on how hard you wind it
up. The worst challenge is min-max-min steps (the
min load case having the worst load-pole position,
against which to re-close the loop from a way skewed
excursion).