Your best bet for a nearly complete list of currently active and populated games is to just check out Mud-Stats:
Sort the games by number of players, make sure online status is set to UP, set a world-type if you only want a certain type of database, and the first several pages of results should be accurate. Not fully complete by any stretch, but it's easier to use Mud-Stats to build your list and then ask people for games that aren't on Mud-Stats.
Here's a suggestion for how to handle keeping the list up to date: instead of a file downloaded and included with the client, have the list of games hosted on your own website and accessed through the BeipMU client. Add a "send report" button either next to each world or at the top of the list, allowing users to send feedback if a world in the list is dead, or if they want to add their world to the list because it isn't in it already. If necessary install a security check so people don't spam the report function and make your lives unpleasant.
That way, even if someone doesn't update BeipMU for a long stretch of time, the list of worlds will always be up to date. Since BeipMU isn't a program you'd use without an internet connection, the list being online doesn't disrupt anything, and being something you can store as plain-text, it won't eat people's data either.