Little "easter eggs" like that are pretty rad! I seem to remember there was a story behind the knife (from the Q&A) but I don't think anything concrete was revealed.
I think the blade on Sion's head is just a random knife token from the war times that has Jarvan the First's blood on it.
Similar to how he now has J1's crown as his new jaw, those 2 objects are only there to awaken Sion's memories and consciousness in his long dead body during the resurrection ritual and not much else.
So I wouldn't say there is a link between SotO and Sion's "ponytail".
Sion has a bunch of easter eggs already. Give him more we might as well give him an easter skin.
Just a few examples: His jaw is Jarvan I's crown, his shoulderplate is Full Metal Rammus' shell.
I always assumed it was J3's flag thingy
You've got my vote. That'd be a pretty cool easter egg, same as the Seeker's Armguard belonging to Sejuani
I fully support this. 
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If you want cheese supports you get
and yes I have played with or against all of these supports (except maybe gnar)
