While we are not a supported system, I would also note that I work on a project which *does* expect `atexit` to work properly, but does not have a working anonymous mmap system due to how resource management works for us.
If ASLR is enabled, that should already mitigate the ability of an attacker to write to this location. While placing these in anonymous pages would be a further defense against cases where a pointer is leaked *and* an arbitrary overwrite is present, this would be an argument for a more complete form of randomization (see also pagerando,
https://lists.llvm.org/pipermail/llvm-dev/2017-June/113794.html ) rather than trying to randomize the location of individual chunks of sensitive data.