I haven't followed Plan 9 for ages, but I'm puzzled why Cox & co wrote "Plan 9", then. However, the point was more about the capability-oriented security in a Unix successor, and how you can use file handles as a sort of cabability without the global namespace. (They're often quoted as examples capabilities in POSIX, but that's ignoring the global namespace.)
In fact, according to sys/src/cmd/auth/login.c it looks like once you've logged it, you can shut the door using the capability device so then it's game over, no more hostowner for you
Plan9 has a completely different security model.
The Hostowner (usually Glenda) is essentially "root" and you're at the mercy of the filesystem regarding file privileges etc.
AFAIK there is no way to "become" glenda.