Frankly, I do not remember. Maybe you want to try and, if it works, open a PR?

I just hit this too. I suspect the restriction is because of this:

The proxy needs to know the size so that it can unmap it.

The corresponding code in OCaml is https://github.com/ocaml/ocaml/blob/061adb721511a5d387e67c56abbfa57c34e9efe0/runtime/bigarray.c#L1083-L1090:

    proxy = malloc(sizeof(struct caml_ba_proxy));
    if (proxy == NULL) caml_raise_out_of_memory();
    caml_atomic_counter_init(&proxy->refcount, 2);
    /* initial refcount: 2 = original array + sub array */
    proxy->data = b1->data;
    proxy->size =
      b1->flags & CAML_BA_MAPPED_FILE ? caml_ba_byte_size(b1) : 0;
    b1->proxy = proxy;

Another possible reason is that file access might be slow, so possibly the runtime lock should be released during Cairo operations. But the file is likely to be an in-memory shm file anyway.

And also, caml_cairo_image_bigarray_finalize calls free instead of munmap, which goes badly.