Any resistor will have a pulse Joule maximum regardless if
rated ot nor. You only get knowledge / confidence with the
rating (you would hope, but are not guaranteed, that a
pulse rated resistor outperforms one that has none stated).
Trimming involves creating a "neck" in the resistor material
slab, which necessarily concentrates power locally and
makes local power-to-blow less than the "bulk" material.
You could of course characterize any resistor you like and
get a power-to-blow vs pulse width curve, margin up for
what you believe is the worst case max pulse and worst
case max amplitude, and go with it. What margin to apply
however is yours to guess.