I found an article about getting seed from horseradish, but misplaced it. The trick was to girdle the stem just above the storage portion of the root, then replant. By cutting off the flow of carbohydrates to the roots, more carbs are reserved to nourish the developing seeds. Similar tricks have been used to get self-seeds from self-incompatible Easter Lilies, Narcissus and Amaryllis (Hippeastrum). I'm attaching a link to a different article that explains the theory.

Karl

Here is a link that might be useful: Reversal of Apical Dominance