Even if the database provider works in  MySQL, most of the stored procedures use SQL Server's SQL Syntax which means they will need to converted to MySQL syntax.

The only reason I find people ask about MySQL is licensing cost of SQL Server.

- DNN can be developed against SQL Server Express (free) or SQL Server Developer edition ($50 I think).

- If DNN is hosted on shared hosting, there are many cheap Windows hosting which offer .NET hosting with SQL Server for as little as $5/month. So with the above cost, it almost matches the cost of LAMP hosting.

- If you want to host on a dedicated hosting solution, you can host with SQL Server Express. The limitation is 10GB per database for SQL Server 2008 R2 and bove, 4GB for lower eiditions.

- Get your own SQL Server license. That's when it becomes spendy.

- Host in Azure with SQL, this might be more effective for large DNN databases.

The problem with using MySQL is that for every new version of DNN, you have to upgrade MySQL database which couldn't take a lot of work and would be error prone.

EF is not going to be of any help if DNN depends on stored procedures. EF works by having entities which represent your business objects and the developer writes Linq statements which EF will convert to dynamic SQL at run time. Until all database access is done through EF and no stored procedures are involved, might as well forget about MySQL or any non SQL Server database.