I want a WhiteListed

Goals

The goal is a WP plugin that could be used on most any other SPLOT/WP site. Right now, out of the box, WP allows blacklisting various email domains – keeping them from submitting comments. But there's not a plugin that creates the ability to WhiteList email domains for commenting on posts/pages.
Use Case: we have WP sites used in classes and WP sites that we want to encourage discussion and comments. We're OK with the pages/posts AND comments being viewed publicly, but we want to limit the ability to comment to only members of our college community. We could do that if we had this plugin by whitelisting the domains *@lcc.edu or *@mail.lcc.edu.
It would be an incredibly easy way to encourage threaded discussion and conversation on any class site by students.

Restrictions

Not really a restriction, but a suggestion. When I've researched and searched for this in the past, I've found some links to snippets of php code that folks say can be inserted into WP to create the comments whitelist capability. Unfortunately nobody seems to know of a plugin.
A plugin would be so much easier for faculty to install. Maybe the plugin can be based on those code snippets? If needed, I can probably dig up those snippets.