Here's a plugin that provides flags in post_meta; only works for non-WP-integrated installs or WP-installs with Buddypress plugin:
https://github.com/NoahY/q2a-flags
Features:
- allows display of 4 sizes of flags, 252 countries, all images included
- creates select field for users to select country
- adds flags to post meta for users
- automatically detects whether Buddypress or non-WP, prevents use for non-BP-WP-integration
- creates user field via button, populates field for Buddypress
- replaces user field on-the-fly for non-WP install
- all country names editable via language files
Screenshot: