The discriminator column has special meaning for Doctrine 2 and thus cannot be part of a relation.
But there is an easy work around. Just add another column and give it the same value that your discriminator column has. The value will never change so it's easy enough to do. You can then of course use your new column in the same way as any other column.
I know having two columns with the same value is not ideal from a database perspective. But from an object perspective, it's no big deal since the discriminator column is never exposed as a property. And it's just the way doctrine works. It wants that column all to itself.