Is it practical to use separate MOSFETs for PWM and direction control?
Such circuits may be usable for motor control, but I really wouldn't recommend one for audio applications.  A typical PWM will behave most linearly when it's near 50% duty cycle; linearity falls off badly at the extremes.  In a class D amplifier, a zero-volt input signal is represented by a 50%-duty-cycle output.  As a consequence the zero-crossing point, which is where the ear is most sensitive to distortion, is where the circuit behaves the most cleanly.  If you try to use an H-bridge simply for direction switching and a separate transistor to modulate the output amplitude, it will be very difficult to achieve any sort of smooth behavior near the crossover point.  This will be especially true if one is driving a reactive load where voltage and current are not in phase.  The behavior of a class D amplifier when voltage and current are out of phase will be relatively clean and consistent when the voltage crosses from positive to negative.  By contrast, in a circuit such as you describe, the transition from positive to negative voltage will trigger a sudden change in how the circuit handles the currents that are flowing in the load at the time of the change.  With a practical load, such currents are likely to exist and to be significant.  An abrupt change in them will almost certainly generate audible crossover distortion.