I agree with Doctor, if it does anything then I hope that it's something that can be gotten once every year during Xmas. One-time rewards are great for cosmetics and lower shiny odds like Mysterious PokΓ©balls, but if the Wishing Stone does summon Jirachi, it would be extremely unfair to new players or players who missed the event to have a one-time PokΓ©mon, especially one that could be so competatively relevant.
If it does summon Jirachi, I hope it can be obtained like Shaymin during LNY, and that older players with the Wishing Stone can automatically select their IVs and nature when they catch their (two) Jirachis, while newer players can grind like normal. Two legendaries with selectable IVs for all the time spent waiting...