Send an email when a game comes available on our system of choice (Steam/Desura)?
Simple option: Ask us what systems we use.
Then when a game comes available (that we already purchased) for that system, email us.
We allow developers to email out updates to all of their customers about Steam keys etc.
Bruce Nicoll commented
Definitely, especially with the mess that is Greenlight. You buy something in the hope that it passes Greenlight and is properly released, but you have no idea if/when that actually happens. If you follow the project on Steam, you're still left waiting a random amount of time after passing Greenlight for you to actually get a full Steam key.