Filtering voltage spikes caused by slow SMPS output regulation
On a transient load dump all of the energy already stored in the SMPS's inductor will have to transfer to the output capacitor.  There's nothing the control loop can do to prevent that. So the output capacitor has to be sized to absorb that energy before the OVP trips.

Aside from that, a poorly designed control loop can cause excessive overshoot.  If that's the case re-tuning the supply or switching to a better supply can help.

I don't know if you can modify the SMPS at all, but your options are as follows:


Ensure the SMPS control loop is optimized.  If so, add more output capacitance or reduce the SMPS's inductor value if possible.
Build a comparator circuit that turns on and clamps the output voltage to a given value before it hits OVP.  
Use a TVS to do the same this as in (2) (both have to be sized for the energy involved) Be careful with the breakdown voltage variation on the TVS, it can be large. 


You can estimate the energy you have to absorb by looking at the peak overshoot voltage you expect to get.  Then you will have to reduce the 1/2CV^2 energy in the caps by the amount of voltage you need to get below the OVP threshold.  By the time the output voltage peaks, most of the 1/2LI^2 energy in the inductor is already in the caps.  

That should give you a starting point for the amount of energy you have to handle, then you can add some margin from there.

The easiest thing to do might be to just add more output capacitance if you have space.