In addition to the above, the body acts as a lossy filter -- or at least, it should. If the pulse is well balanced (AC), the effect is mainly heating. If unbalanced (like an electrostatic spark), all the high frequency AC content again is filtered out (and absorbed as heat), however the DC component still develops a voltage drop across membranes, causing electrolytic transport, ionic movement, depolarization, all that. Case in point, you don't feel the streamers from a Tesla coil (give or take intensity, largely because of heating AFAIK), but an ESD spark is quite a lively experience.
There may be studies about how much charge, at what rate, is necessary to do those things. I think the response is not quite a constant-charge asymptote, for short pulses? That is, say you discharge a capacitor of given value and voltage, through 100 ohms, or 1k, or 10k, etc.; the same charge is delivered, but at different rates. For rates faster than the body's filtering time constant, the effect should be ~independent of rate (so maybe 100 ohms feels the same as 1k, but 10k and up feels weaker and weaker).
The filtering effect itself, is probably not a dominant-pole effect (i.e. that would give a simple asymptote like the above), but probably more like diffusion, i.e. dependent on sqrt(pulse width) or something like that. Which in turn arises from current flow through a resistive bulk medium. In other words, skin effect, yup. That would be a physical explanation for it, anyway.
Anyway, studies are what you're looking for, to put numbers on those general ideas. I think pulses of some ms have been measured, though I forget if µs too?
To clarify "objectively safe", for my part, I would want that to include cardiac function as well as physical damage. ESD apparently doesn't cause much damage; presumably it does, but it's microscopic and infrequent enough to ignore. And people don't usually fibrillate from it, AFAIK. Probably, somewhere in the HBM 10-20kV range is a reasonably-safe marker, in that region of pulse shape/amplitude/length.
Tim