...never substitute a dried/powdered herb when fresh is called for in a formula. In this case, no type of horseradish that is dried packs anywhere NEAR the power of fresh. I'd have to do a bit of studying/researching on the herbal qualities on wasabi vs horseradish before I'd consider (or recommend) switching/substituting. I know that wasabi root is extremely perishable, and that most wasabi pastes are "blends"...'would probably be pretty hard to find organic wasabi roots.
Since they're in the same basic family and have similar actions & effects, if fresh wasabi root was the only thing one could find, then it would likely be better than nothing.