I don’t think bytes (even with finalizer) would work in this case because the GC might’ve moved the value (perhaps several times if the heap is compacted), and I’m not sure there is a guarantee that the old value would’ve been completely overwritten by other values: minor to major heap move would leave behind a copy in the minor heap, and during compaction in the major heap it is up to the libc allocator what it does with freed memory (it may keep it around and reuse it instead of immediately giving it back to the OS, depending how big it is).
A bigarray might work, and might be the preferable approach (it doesn’t rely on any GC or libc implementation details and gives you full control over scrubbing), but comes with the drawback that none of the string processing routines would work with it, and you might have to create temporary bytes that need scrubbing (although that can be avoided if everything using the secret is converted to use bigarrays, or the API of this SecureString module).