Dual licensing is rather problematic both from the community perspective as well as a business arrangement. Over time, even projects like MySQL have been getting more revenue from traditional support and services as opposed to selling proprietary licenses. Trolltech changed from GPL to LGPL for Qt as well. What happens if there is a very good piece of AGPL licensed code that the author is unwilling to submit to you under your copyright license agreement?
Why not just require that all contributions must be permissively licensed and skip everything else. IIUC, that's what drizzle, a fork of MySQL does and it seems they are very successful is getting community contributors compared to MySQL.