Hi Neo. As always, interesting questions.
I am certainly no expert on this history, but here is a Wiki link that might be helpful despite certain lacks of citation.
http://en.wikipedia.org/wiki/Srivijaya
From what i can gather this Buddhist kingdom finally fell to the Mojophait kingdom of Java in the late 13th century. But it had much contact with Java before this, especially with the previous Singhasari kingdom. I would say that it is quite possible that the keris made it's way to Sumatra during these times. They would have certainly at least come in contact with the keris. Whether it became an intrinsic part of their own culture i could not say. But this would mean that the influence of the keris there might have come from it's original Hindu culture, not Jawa's later Islamic one.
