Seriosity.com thinks it has a solution to email overload. They offer firms a signaling service based on an artificial currency ("serios"). Potential message senders are endowed with a fixed supply of serios per week, and they can spend them to indicate their estimate of the value of the message. The company demo suggests that managers may not read underfunded messages, and will read highly endowed messages first. (It isn't clear what the equilibrium would look like in a dynamic environment.)
But creating scarce signals is not a bad idea, see e.g. the mechanism for Signaling on the Economics Job Market , which allows each candidate to send up to two signals.
From their website:
"Attent creates an economy with a scarce new currency (Serios) that enables users to signal the importance of their outgoing email by attaching value. Recipients can use the Serios received to prioritize their attention to messages, and in return use their Serios to assign appropriate weight to their responses."