What is the feasibility of using an electrically charged (as in not neutral) plasma as the working fluid in a turbine where the blades are permanent magnets rather than hydrodynamic fins
You are in a vacuum? So the plasma flow is, essentially, a current in vacuum.
The bad news: the turbine won't work
A charged particle does not exert useable force on a static magnetic field, nor does the static magnetic field exert useable force on the particle. It's a consequence of the equation for the Lorentz force. The plasma current will be curved around the magnetic field without losing energy, as it happens with the solar wind when it impacts the Earth's magnetosphere.
You might build something like a Crookes' radiometer running on ions instead of a thermal exchange, but the plasma would rapidly ruin the blades.
The good news: you don't need a turbine
A plasma flow in vacuum is a current. By intercepting or modulating the flow you can make it a variable current. A variable current will induce a variable magnetic field, and a variable magnetic field can be used to produce energy at the expense of the kinetic energy of the charged plasma.
Your machine is essentially a hollow cylinder through which the variable plasma flow runs. The inner walls of the cylinder could be covered with ice, or some low-melting non-magnetic metal (GalInStan, Rose's metal, Wood's metal, mercury) in order to easily counteract erosion by stray particles. Or you could re-sheath them when needed.
Inside the cylinder you have large superconducting coils running all around the cylinder, circuiting around the incoming current and making a large solenoid. Every oscillation in the plasma current will induce a voltage in the solenoid; you've essentially built a large and inefficient transformer in space, but if the plasma flow comes from some large natural phenomenon, inefficiency isn't really an issue, and you get lots of energy for free.
Plasma jet engine
With a bit of handwaving, you might imagine a first stage that will both compress the plasma flow and induce in it a series of rapid oscillations through three-dimensional vortex shedding. The oscillating plasma flow can then be exploited using the cylindrical transformer described above.
The overall scheme would be somewhat similar to that of a scramjet propulsor.