Where to place a capacitor to prevent arcing in a brushed DC motor?
It depends .  This is not a simple yes/no answer.

It is better for source EME or EME but maybe not load brush commutated temp rise to absorb arc energy if a brushed motor under full starting current (x10).

The schematic trace and cable impedance/ inductance is not shown so loop current transient flux and EME is not known. Thus in 2D simulations it may show where current pulse rises on conduction and current decay during dead time. 

But how much interference is generated?

Simulations may also show but CM inbalance and direction of arc current during commutation and ripple current ^2 *ESR =Pc of Cap and thus temp rise and drop of MTBF of 50% approx per 10’C rise. 

But it won’t do like COMSOL and graphically show crosstalk of mutual induction from interference to high impedance signals.  (Advanced 3D EM physics)

For extreme cases RC//C can absorb more energy and filter EME noise better  and possibly affect motor contact temp rise   good or bad depending on snubbing or cap shunting making it worse.  

Cables are the biggest problem or  antenna source of common mode EMI or EME as  some prefer to call it. But motor contact degradation from arc energy dissipation is also a key design factor and cap ripple current rating margin.

But if you don’t care about EME , the motor may reduce arcing during V=LdI/dt turn off duration but increase current during V/ESR=CdV/dt for a shorter duration but possibly  higher current unless RC// smaller C with CM choke on motor cable.  So both methods may be considered depending on layout .

Diodes also add some capacitance in addition to Coss.

The other concern is which diode has lower ESR ? Externals or internals to MOSFETS, If C of diode increases as ESR drops with rising Pd rating , you already have some deadtime load capacitance and current direction from the diodes is the same as previous conducting FET so the diodes need to be close to switch RdsOn for steady voltage from current FET switch during deadtime. 

Thus if done right , keeping the Commutation current to the motor from the switches to diodes carrying the current will be decayed by the lumped capacitance across the switches more effectively than across the motor, because the cable is the noise antenna.  This  also demands that the driver to gate and motor ratio of RdsOn is low to drive the low ESR diode capacitance and EMI shunt Cap &lt;=0.1uF according to DCR of motor current limit from winding resistance. Typically the ratio >20 or &lt; 5%loss with ...
ratio =Motor DCR/ RdsOn of drain and
=.    gate driver RdsOn / drain RdsOn  for rough Rules of thumb 20 is minimum not 1000 while some are 100:1