Hmmm...keep in mind that brine shrimp are enriched by feeding them before freezing. Spirulina/vitamin-enriched brine shrimp are fed the aforementioned food items, and thus have the spirulina/vitamins in them. Soaking the brine shrimp in something like selcon will help coat it in the enrichment solution, and surely some of it will enter into the food item itself.
I think that mixing in spirulina will be a waste of time/add extra waste into the tank. A better idea would be to take some dried seaweed, cut it into small pieces, and while defrosting the brine shrimp, soak the nori in with the brine shrimp and whatever supplement you add, then feed the entire thing to the tank.
