I don't know much about creating Minecraft mods but I suspect there might be a hook for when a block is placed? If so, this would work as a simpler smaller-scale replacement for a construction task. There are some obvious shortcomings here (a block could just be broken and placed again, as keeping track would be too expensive) but it'd be a nice thing to have for small-scale situations where you can trust that the player will perform the task.

(I'm still using 1.7.10, so if this feature is considered, I hope it makes it into the relevant BetterQuesting version)