UPDATE: I had zeners in my circuit for a while, but I'm starting to think they may not switch fast enough or whatever to keep up with the switching of the boost converter in order to drain the excess energy away.
I was thinking of using Avalanche TVS diodes. Except...those are designed for transient pulses aren't they? So their steady state ratings aren't given at all. THey also have less accurate breakdown voltages than zeners. Does anyone know anything more?
NOTE: Remember that the load is a pulsed power load so the converter spends most of it's time charging these capacitors up to full voltage at which point no more current is drawn by the load. Only occasionally are the capacitors drained to provide power to the pulsed load. So when the caps are fully charged and the load hasn't drained them yet, the boosted energy from the converter's inductor has to go somewhere...