Adding flash on clash to a saber without it is really simple. If you buy one with Premium sound and a buck puck, you don't even need to worry about the resistor thing, as the puck will supply the power you need to each LED. As you say, all you have to do is upgrade to a multi die LED, then wire up all your positives and negatives, splice the pos and neg for the mains to the wires, then run the neg for the flash back to the board and solder it to the only post without a wire on it on the Obsidian board. Easy peasy, with minimal soldering skill required.
Though, even if you have basic sound, I'm sure it's not any harder. You just need to know what resistor(s) to use. Wired in parallel, you don't have to worry much about power drop along the LEDs, until your batteries start going dead.
