One thing to consider is using addressable LED strings. This is exactly what I'm working on for a project for a show next year. We're using 50 RGB LED strings which are daisy chained together to make a longer string. We're still working out the power needed, but since the lights are never on all at once, we won't need some bulky and/or heavy pack. For a single string of 50, we can run them on 6V for hours. So even if we use 4 strings, and strap four 4-AA packs onto a belt, it won't be heavy for the wearer, but we're working on finding a different solution. The strings we use only require 2 pins (MOSI and CLK) so as long as whatever controller you're thinking of using has SPI, you're golden.
As for having "pluggable" pieces (such as the addition of a belt), that just requires some thought on how you want things to work. You could possibly make it so it's at the end of the string, or you can have a controller with multiple SPI outputs and use a second port for the belt ... different options here. You can also ignore SPI all together and go with bit banging and you can use any pins at that point.