So you are looking for something like a string of say 255 LEDs, where each LED has its own address, you would connect all with common power, ground, serial-input, each would listen for its address and then act accordingly? Could have each buffer the input and pass it along to maintain signal integrity.
5 meter, 195 inches/500cm, you could have an LED/board about every 2 cm.
Send 2 byte message - 1st is address, 2nd is data telling it what to do.
Maybe have a couple addresses that are global commands.
This kind of thing
http://www.bliptronics.com/item.aspx?ItemID=88
but with white only?
Maybe have each board have an LED, current limit resistor, 8-pin ATTiny85, and a single buffer chip.
or make your own with 2 NPN transistors (back to back inverters)