This one's not really sneaky, and I'm sure everyone tried it as soon as they went into Avalanche, but if you're standing outside the little shortcut through the middle (where there's a sniper rifle, battle rifle, two plasma nades, and overshield - aka- MLG Heaven!) you can toss nades into the mancannons in the center, and they'll land right where the flag and bomb plant sites are. Plus, the plasma nades respawn pretty quickly.
Also, thanks to garbage KSPR for showing a ton of jumps on Blackout. I found another one though. You can jump from BR tower all the way down to bottom lift. The only thing is, you don't really have enough momentum with the bomb or flag, otherwise it would make for about a 15 second cap.