Have you guys thought about overloading the DDA put methods to accept nullable values? Here are the extensions method I use for int?: 

 

public static void PutInt32(this IVistaDBTable tbl, int index, int? value)
{
    if (value.HasValue) tbl.PutInt32(index, value.Value);
    else tbl.PutNull(index);
}

public static void PutInt32(this IVistaDBTable tbl, string columnName, int? value)
{
    if (value.HasValue) tbl.PutInt32(columnName, value.Value); else tbl.PutNull(columnName);
}