The problem as per the Phusion blog article is that the runit and possibly daemontools equivalent commands don't set all env vars: HOME, USER, GID & UID, it only calls setgid() & setuid() IIRC. (envuidgid only sets UID and GID[0]) This causes breakage for lots of apps that end up inheriting root's env instead. :(
Here's the go version I wrote, so there's no need for dep on Python/Ruby/etc on the target system:
setuser USERNAME COMMAND [args...]
# how to build it
go get github.com/steakknife/my_init/setuser
go build github.com/steakknife/my_init/setuser
# creates setuser exe here
Another plug for daemontools-encore (last one I promise). The setuidgid(8) program in classic daemontools doesn't set supplementary groups, and a lot of people I know just end up just using sudo in their run files. If you use daemontools-encore though, you can just use `setuidgid -s` instead.
PS. Really, daemontools-encore should imo be called daemontools 1.x, but you know how djb feels about forks of his projects...
[1] http://cr.yp.to/daemontools/envuidgid.html