Socket.io (node.js) on an EC2 array of servers; alternative to redis store for discovery and persistency?

I'm running socket.io/node.js on my EC2 auto-scaling array of servers. As soon as I have more than 1 server, there is the obvious problem where the socket.io connections need to be shared between the servers. This is where you'd normally use the redis store plugin that comes with socket.io.

Unfortunately, I use MongoDB (not redis), and the cost of adding another database to my stack would be prohibitively high. I tried using the socket.io-mongo module, but it absolutely killed my servers (huge CPU usage).

Is there any other way to share the socket.io session between EC2 servers?