I had briefly considered creating an Oracle data layer for mojoPortal, since that is the primary database we use at our organization.
To get started, you'd have to duplicate each mojoPortal data layer assembly and update the SQL calls for your target database syntax. I'd recommend basing your new data layers on the MySQL versions for ease of conversion. The MS SQL data layers rely on creating and maintaining a separate procedure for each database call, whereas MySQL builds up and uses raw SQL statements. MS SQL may have the security edge in a large environment where SQL access is locked down and only granted at the procedure level, but that's a lot of extra development overhead for a typical site.
Second, you'd have to copy and update all of the SQL scripts under Setup/applications/[application name]/SchemaInstallScripts/[dbname] and Setup/applications/[application name]/SchemaUpdateScripts/[dbname]. There are a very large number of these.
Finally, for every single release of mojoPortal going forward, you would be responsible for duplicating any functionality changes made in the data layers and in the upgrade scripts into your own code. This is the primary reason I gave up on the idea of building my own and using MySQL instead.
I hope this helps,