In unrelated news:
I have now run 2 sessions of Cross system games. It is looking like this is going to be a annual tradition for me.
One with wizards from L5R, dnd 4e, Mistborn and nWoD (last year)
the second with assassins from L5R, nWoD Heros, Dark Hesesy and Zorcerer of Zo (this year)
I don't remember what I used to run the NPCs last time, but this time I used what i am calling the D12 Master System. Which is sometimes rolled as a dicepool (Success on 10-12), sometimes rolled as a d% (Success on >100%)