usernames installed by packages

[Nick <debuser@acrasis.net>]

Having failed in my web searching, I ask for wisdom...

Suppose I have a web app where people can register, choosing a
username.  The app takes the username and creates an operating system
account on a Debian server.

Naturally I don't want to let people choose 'root' or 'nobody' or
'www-data' or 'bind' or...

How can I get a list of all the possible usernames that might be
required by packages on a server?  Or is this a misguided way to do it
and I should instead modify the user's choice by applying a prefix or
some such?

Thanks,
-- 
Nick