It's possible but takes a LOT of work and
TESTING. I know because I wrote a full DAO based on Apache Torque (cause I'm one of the Dev's there and my app used it).
One thing to consider before going down this path is that JForum 3 is migrating the DAO to Hibernate (despite trying to get Raphael, et all to use Torque.. sigh). So, what you do for 2.1.x may or may not work with 3.0.
That said, basically, you will need to implement all of the interfaces in the net.jforum.dao package and make your own extension of the DataAccessDriver class to call them. Of course, you'll need to test that all the methods return or store the information as required.
I think the hardest part will be the various queries that return information that is a combination of various tables (e.g a lot of joins, etc). Most REST DB implementations I've seen are pretty table/view record based and don't do joins well.. yours may be different..
[originally posted on jforum.net by monroe]