1. With the calm mood being the only active mood in the server, this idea would currently be way too cheap to earn more cookies, that it shouldn't even be considered as a challenge.
2. Sure that there could be an addition of limits on how much blocks that one is able to place/break, but consider the players who would take advantage of this. For example, if a player turns on iron man and is limited on the amount of blocks they would place/break, then they would simply pillar up the the highest capacity that the lava amplitude can reach, and boom. Easy extra cookies for the round.
3. What would happen if they lose the challenge? In classic, it works because when they lose iron man, they not only lost cookies, but all of their lives and a super cookie as well. And trust me, those take some effort to make up. Especially if you had a stockpile of lives (like 100+). In LSMP, all you have to currently lose is an amount of your cookies, and that really isn't an effective risk at all since cookies aren't hard to make up.
As Raz said, this shouldn't be in the priority list. It's a nice idea with the challenges that it provides, but I would wait until LSMP develops further into its content, and then this extra feature may be considered at that point of time, depending on what content is released.