This article's list of privilige dropping tools is missing setpriv(1) from util-linux, which is the only one actually installed on my system.
If you have BusyBox installed, you have runit's chpst and setuidgid built in to it. And most Linux distributions not only package that but also package one or more of these toolsets; most often daemontools and runit, but nosh has an Arch package for example.