If a common mode choke (CMC) is added between PWM signal and PWM return without additional chassis return path, will it act as a common mode filter?
It is exactly like that. Differential current (the load current, out of phase from the coil point of view) pass more or less unimpeded. Common mode noise from the drivers get attenuated (in phase from the coil point of view).
However there are subleties: the CM choke does not suppress (or, at least suppress much less) the PWM switching noise itself, since that's is differential mode. There are specially designed chokes that do also that or, simple, you put a simple inductor (called simply choke, in this context) to attenuate these. This often is magnetic field (for a number of reasons there's an electric field component too) and can be attenuated also simply twisting the wires.
Also your CM noise is actually returning thru the parasitic ground (there's always a ground path to earth, even is maybe only about 4pF) since it's traveling from both the wires and emitted from these. This is only an electric field thing due to capacitive coupling to earth.
In any case be wary of the current rating of the chokes since they not only do heat up but also have a magnetic limiting value due to core saturation.