Basically what I need is a simple tool that helps me finding the spots that I need to illuminate because there's a chance of mob spawns there, a way in which this could work is that the player can toggle the detector by going like /ld or /lightdetect or /spawndetect or whatever; when the tool is working, you get a message if you enter a safe zone (walking on slabs or light over 7, or any other viable condition) or when you enter a spawn-enabled zone (light under 7, spawn-enabled block under your feet). That should help you put torches down efficiently. You could even have a tool that, provided a radius, placed sand blocks in the area near you that can spawn mobs, you then simply remove the sand and place torches where necessary to prevent it from repeating, even better if the "magic sand" trick were to actually use sand from your backpack, and maybe a "casting ingredient" like I don't know, redstone, or gunpowder, or feathers, or bone. Rules for mob spawning: http://www.minecraftwiki.net/wiki/Spawn#Mobs P.S.: it should be unusable in the nether, for obvious reasons.
Hm that's kind of a plugin which I planned. While walking the plugin replaces every near block with green wool, if spawning is not possible, yellow wool, if neutral mobs are able to spawn and red wool if spawning is possible for aggressive mobs. But I'm waiting for the new 1.8 update which changes a lot in the lightning system. Maybe someone else would like to write it now