I'm thinking of using a text adventure as part of a larger, ARG-style puzzle. Is there any way to generate a code or track that a player has beaten a game? They could even just enter their "name" in a form at the end. Anything.

If the player has an online connection, you could use SetExternalFileData(phpscript, string) to call a custom PHP script and write the name or an ID to a file on your own server.