I'm attempting to add concurrency row version column to an existing table. I know the column needs to be an int, but what about NULLs or defaults? What is the BO designed to handle? An initial Null value or a default of 0?

And one more question...how do you configure the row version column in the BO? Readonly? Null handling?

The row should be initialized as 0 and the BO will handle the rest.  You do not need to do anything special other than setting the BO to use row versioning and set the row version column on the BO.

FYI, you can find more information on this in the help docs:

Business Layer -> Configuring a Business Object -> C.R.U.D. Settings