[remulasce]

As per [R66]:

A noise filter may be wired across motor leads or PWM leads. For the purposes of Inspecttion (sic) and rules compliance, such filters will not be considered custom circuits, and will not be considered a violation of Rule [R49] or Rule [R65]. Acceptable signal filters must be fully insulated and are:
A one microfarad (1 µF) or less non-polarized capacitor may be applied across the power leads of any motor on your Robot (as close to the actual motor leads as reasonably possible).
A resistor may be used as a shunt load for the PWM control signal feeding a servo.

So yes, perfectly legal.