Gravitational waves produced by inspiraling black holes have a similar inverse dependence on mass that the strength of the gravitational field outside of the event horizon has. For example, look at how mass decreases with increasing frequency in this graph. Presumably, if the mass of the black-holes is sufficiently small the peak frequency will become high enough for the gravitons to pair produce, with each-other or off of CMB photons. What is the largest mass at which identical colliding black holes could pair produce, in principle, two particles of identical mass?
Granted, in a real situation there will be difficulties in establishing the energy threshold based on circumstances (graviton + CMB photon will require a really high energy graviton), geometry (graviton + graviton will likely be colliding at an acute angle instead of head on), and factors about which I am unaware. So let's just assume that the highest frequency gravitons produced by the collision can meet head on in finding this mass limit, to keep things simple.
Also, assume this is for electron/positron pairs or neutrino/anti-neutrino pairs, since if this can happen it should produce some interesting neutrino flashes.