This seems like a strange limitation. If you were using TopLink Essentials I would recommend that you map the column as an @Basic and mark the @Column as updatable=false, insertable = false (just to ensure there are no conflicts between your code and the Persistence Provider). If you didn't mind using native TopLink you could simply add a 'QueryKey' for the discriminator column and would not need the mapping at all.
Otherwise you could explore Native queries and SQLResultSetMappings.
--Gordon
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