I asked a colleague (R. Jokipii, a retired expert in this field) about this, and here's my summary of what he told me.
Since the spacecraft separates the plasma outside from the people inside, any people inside would require no additional shielding from the plasma—it's only a significant threat through contact. He emphasized that plasma itself is so tenuous, even shocks would not present significant dangers to the spacecraft: there's simply not enough of it there to be a significant threat.
"Even with the significantly lower intensity of galactic cosmic rays at Earth's orbit, astronauts would exceed current lifetime limits for astronauts within about 6 months when outside of the Earth's atmosphere and magnetic field. At present, there is no practical method to shield them from the galactic cosmic rays. Of course, the situation would even be worse at [location of] the Voyagers."
So, it's the radiation that would be the danger for astronauts on manned probes—not the plasma.
He pointed me to a paper by Eugene Parker, an even more well-known plasma physicist who studied these interactions ("Shielding Space Travelers", Nature, 2006). The point of the paper is that cosmic rays pose the greatest threat to space travel, so shielding against these is critically important for any manned space exploration.