What about decorative light blocks? They'd behave just like note blocks. You right click them to change the color. You left click them and they light up for a split second and produce light! Colored light if possible, but not really important. Hook them up to red stone circuits... endless possibilities! Or maybe you have to use the right recipe for certain colors. Maybe start with red, yellow, green, and blue blocks (give some more meaningful uses for lapis blocks or dyes!)
pulsar lights that go down a hallway..."neon" signs...giant x-mas trees with twinkling lights...or just forced light updating while you're away from your wheat farms? just a couple of ideas.
I tried making glowstone, piston & redstone lightswitch, and it became a disaster with my redstone skills.
I find the idea interesting, but perhaps it's too big for implementation right now. It should be a mod first, so we could see how it works out.