You can shit in your hands and clap. I would not advise it though. No I would think common sense would tell you pure worm castings will have way too much nutrition for a seedling and it may sprout but its opening it up to all sorts of diseases too worm casting are not inert like coco or rockwool so it could be very harmful for your seedling though as stated you can do what you like :) its your seedling best of luck brother :) 👊