What is the best way to generate a sample/test version of every email that can be produced by a Rails application (for example, so that someone else can look them over to make content changes)? If there isn't an easy way by default, are there any gems or easy code changes that would facilitate doing this in the future?
There isn't an easy way or a gem that does this. The reason is that emails can be implemented in a variety of different ways. My recommendation would be to either keep your email template files in a separate location - so that they are easy to retrieve and send to someone, or to use an external API for sending your emails, such as MadMimi, that allows you to keep your email templates on a separate website.