Would a charged point particle (or a collection of equally-charged point particles) "feel" electromagnetic acceleration? The Equivalence Principle says that free fall is an inertial reference frame, and that you can't distinguish gravity from being in an accelerated reference frame.
One of the ways I understood this is that, because of warped spacetime (metric tensor isn't Minkowski anymore), the proper acceleration (the magnitude of the 4-acceleration) in free-fall is zero. And since the proper acceleration is the acceleration that is "felt" by an object/accelerometer as it's accelerating, it makes sense that it would be zero when your situation is indistinguishable from "having no acceleration and being in an inertial frame".
However, think about a charged point particle in a uniform electric field. Or perhaps a group of charged point particles, all with the exact same mass-to-charge ratio. The proper acceleration isn't zero, it's qE/M. And yet, an accelerometer made up of only particles with the same mass-to-charge ratio wouldn't read anything, because all its constituents would accelerate at the same rate.
Does this mean that proper acceleration isn't always reflective of what acceleration is "felt" by a system?
 A: 
And yet, an accelerometer made up of only particles with the same mass-to-charge ratio wouldn't read anything, because all its constituents would accelerate at the same rate.


Does this mean that proper acceleration isn't always reflective of what acceleration is "felt" by a system?

In Newtonian mechanics this indeed would be true: it is impossible to detect the presence of a uniform field producing the same force per unit mass in all bodies.
However, if we have several charged particles with different velocities comparable with the speed of light, we would notice that they accelerate in a uniform electric field differently. So, special relativity provides us with varying effective charge to mass ratio, which would allow one to deduce the presence of uniform electric field.
A: 
The proper acceleration isn't zero, it's qE/M. And yet, an accelerometer made up of only particles with the same mass-to-charge ratio wouldn't read anything, because all its constituents would accelerate at the same rate.

All of the charged particles would move the same way which can't reveal their acceleration in their rest frame only if they started with the same velocity and weren't interacting with each other.
In reality because they are accelerated in a frame where Maxwell's equations hold, they will produce EM forces on each other which will affect their motion and this changed motion will reveal they are under action of external field even based on positions and velocities of the particles in the rest frame of the system.
