(Discussed the welcoming culture in Phabricator at Lyon Hackathon 2015 where this was a topic again.)
bugzilla.mozilla.org displays "(New to Bugzilla") in the Comments section next to the names of recently registered Bugzilla accounts, shown to people who are in editbugs. (Downstream Mozilla Bugzilla code ; background discussions)
This could be a helpful reminder to be patient and friendly to our new community members and contributors. But it could also be branding users as "newbies" who might not like that tag.
Implementation could be similar to the "Disabled users" circle shown in front of a user name, and checking via the dateCreated value (currently not exposed) of that account in phabricator_user.
Files that potentially need code changes (or not) are:
Upstream comment about using badges for this (=not applicable): https://secure.phabricator.com/T6526#116150