Using Parameter Maps to implement Data Masking

Here's a real world example of using Parameter Maps to implement data masking as well as disadvantages of the below solution.

Requirement: Data is stored in 2 different schemas with the same table names. One schema contains Secured Data and the other Unsecured or masked data.

Users have access to either Secured Data or Unsecured Data. This information is stored in a security table. Users need to be dynamically pointed to either of the schemas depending on their security level clearance.

Solution:

Step 1:The security table contains the following columns - UsrID, HasSecurityClearanceFlag.

Step 2:In FM create a data source query Subject called SecurityLevel based on the above table.

Step 3:Create a Parameter map called Usr_Access based on the above Query Subject with key set to UsrID and value set to HasSecurityClearanceFlag.

Step 4:Create a model Query Subject called SchemaName with columns - UsrID, HasSecurityClearanceFlag, Schema.

Schema -

case #$Usr_Access{$account.defaultName}# when 1 then 'Schema1'
when 2 then 'Schema2'
else 'DefaultSchema' end

Step 5:Create another Parameter Map called Schema based on the above Model Query Subject with key set to UsrID and value set to Schema.

Step 6:In Query Subjects that need to be dynamically re-pointed to the schemas modify the SQL as below:

Select * from #$Schema{$account.defaultName}#.TableName

Disadvantages:

Modifying the Data Source Query Subjects results in Metadata Callbacks and this is a cause of concern.

I am still trying to find a viable solution wherein there are no callbacks and yet we can achieve the above security requirement.

Any suggestions?