Clamp it before drilling the hole longitudinally along the length of the crack from inside the mortise, to subtend the crack. Drill the hole big enough so there's room for the plug and epoxy to not prevent them from keeping the crack from not closing tight when re-clamping the glue up. Gluing the plug in along the length of the wood grain is way stronger than gluing the long grain plug across the grain of the stock. Always make a joint to be long grain-to-long grain. Long grain-to-cross grain is always weaker.
Truth be told, no plug is really necessary if the sides of the crack are clean to allow good glue bonding. The glued joint will be stronger than the surrounding wood. If it's gunked up then definitely drill and plug.
Yes, this...
