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Would those tools be primarily proprietary, or do some live in the open-source world too? I’d be interested in the techniques they use to aim for realism there.

Admittedly I haven’t made much of a search, but various search engines and autocorrects seem really convinced that I mean “defragmenter” instead of “fragmenter,” despite myself…

It's usually custom scripts, I'm not aware of a standard tool. You fill up the file system and then do lots of operations that cause fragmentation.

Actually here is a recent paper discussing this (although the basic concept is much older):

https://arxiv.org/abs/2401.08858

Arch Linux Wiki on stress testing: https://wiki.archlinux.org/title/Stress_testing

This makes me wonder at the absurdity (yet plausible implementation??!?!) of a tool to test something like ZFS filesystem limits, as they are quite gargantuan in many possible measures.

On CoW systems, file and block deduplication and writes to the middle of a file will do that for you.