How should unidentifiable breakers and fuses be represented in arc flash studies?
I would start out by assuming that they provide no additional protection. If the result of that (overly conservative) analysis determines that the fault energies are O.K. then you should be fine.

If it turns out that the conservative approach results in too great of a arc energy, you'll have to dig further.

If it's only one or two breakers, perhaps it's worth replacing them with a device you do have data for. If this is across many devices in multiple boards, can you remove a couple of the breakers from service and use an inject test set to characterise them? Then use the characterised data for the arc flash study.

Finding an acceptable solution will most likely depend on what will be acceptable in your regulatory environment and to your customer. Personly, I'd avoid making assumptions on equipment that I can't trace the data on.