There is already a plugin named request_username (from cash). But it was for 1.7 branch. You can try it.

@Purus : yes, its already documented. Check the documentation.

Thanks Webgalli - can you help me with the cron command I'd need to use? Or point me in the right direction on the Elgg documentation?

http://docs.elgg.org/ - do a search on "cron". There are docs for setting up cron to hit your site at periodic times. Then take a look at the log_rotate plugin for an example of doing something on a schedule. Then look at the documentation for elgg_get_entities() in the code for how to get all messages older than some time stamp.