Typically this is explained by the saying, "current kills."
It's not the charge (or potential above ground) that a body attains that hurts biological systems, it's the current that flows through them and either 1) heats them or 2) disrupts important electrical signals in the body.
Heating damage occurs and can "cook" (cause 1st, 2nd, or 3rd degree burns internally or externally) portions of the body where the current flows.
Electrical disruption is the more deadly of the two, commonly. The heart depends on regular electrical pulses from the sinoatrial node to contract not just rhythmically, but for the entire heart to contract at the same time. If an electrical current is induced across the heart at the wrong time, or across a weak heart it may lead to atrial fibrillation. This is the most common abnormal heartbeat pattern the heart ends up following when presented with such an electrical interruption, and it's extraordinarily inefficient, leading to low blood oxygen. AED (automatic external defibrillators) are designed to detect this abnormal beat, then apply a specific electrical current and waveform to the heart to give it a good chance of regaining normal beating. With very high voltages, we can discuss the muscle contraction that takes place, and the possibility that this would cause the person some damage depending on their location (ie, throwing themselves backward into a concrete wall due to a sudden jolt), but this wouldn't be the case for the situation at hand.
While heating does occur any time there's any current flow (except in a superconductor, theoretically), it takes a lot of current to cook organic materials.
While the heart can react to electrical currents, it's not very sensitive.
Thus it's the current that flows through the body, or the heart in the case of fibrillation, that is important.
The charge required to bring a human body up to the potential of the circuit they are touching is very, very small. Effectively the human and ground becomes a capacitor with an insulator between them - a small one on the feet, and the air as insulator elsewhere. The capacitance would be very small as the insulator is neither thin nor large (the closer the two conducting surfaces, and the larger the surfaces, the greater the capacitance).
So it would only take a very small amount of current to charge up the body, and that current isn't significant enough to cause either heating or electrical signal disruption.
Further, from the point of contact, the charge would rapidly radiate throughout the relatively conductive body, meaning that the area of greatest current is the contact point, but after that no other spot on the body receives any significant amount of current. Even if you were able to increase the capacitance significantly, and you chose a very, very high voltage, thus forcing many more electrons into the body, the only part that would become damaged would be the heating damage at the point of contact with the conductor providing the charge. You might get a small electrical burn there.
So the situation you describe could only generate a very, very small current to bring the body up to a specific voltage potential, and thus would be unlikely to produce any damage.
That being said, don't do it anyway. Take all reasonable precautions.