I think if this were to be implemented, an amount of mana received per accepted map would make a lot more sense than a block-by-block thing (how would that even work anyway?) People build at very different speeds, so someone who is copying a build from a singleplayer world would get a different amount of mana than someone who is building from scratch in the plot. Also, not everything that is built is submitted, let alone accepted - you're not contributing to the server with a rejected map. Getting say 500 mana when your map is added to /warp beta makes a lot more sense.
However, I don't back this idea. Building should not be a means of gaining mana. Knowing that your map is being played by hundreds of strangers is more than enough incentive, and the Mapper role is an added bonus.
You said (in other words): "Mana should not be incentive for building".
I think that a block by block xp like incentive would add an additional rewarding system that would make building feel similarly rewarding as the parkour, making more people want to create builds, even if just for fun. This is the spirit that parkour has. The logic is, I want building to be more similar to the parkour, In parkour, you get a mana incentive every 1-5 minutes or so (or a bigger one for harder maps). And it makes building feel a bit emptier and less worth doing than parkour does to the average joe. I'll say it again with less detail: I want the incentivized process of building to be more similar to the process of doing parkour. Cause right now I sense a pretty significant imbalance.
You said: You're not contributing to the server with a rejected map. My answer: Like in parkour, where it is the process of doing parkour, not the result; it is about refining the process of building, which is what I think is lacking. Not the result; seeing thousands of people use it and eventually becoming a mapper.
You asked; how to make this work? Well. I don't need to answer this, as the devs are probably more equipped than I to do so, but this would be one of the methods of implementing it I can think might work:
Make it so the server checks every half an hour or longer, sees how many blocks have been added to the plot, and then adds a very miniscule amount of mana for the time spent. Mana incentive would not be much, but it would make the parkour and building worlds blend more in feel, which I think would be better.
Exploiting.
The only way I see this way of earning mana would be exploited would be if somebody used a massive number of hacked clients to automatically fly around placing blocks indefinitely, with a randomized pattern, with certain time gaps between sessions of placing blocks.
But then again, we could have people making parkour bots that know how to go through all the parkours too. It would just be a little harder for people who make hacked clients, making it less likely to happen. I think that if people ended up logging in with 20 bot accounts with VPNS and started randomly placing blocks without making a coherent parkour, we would figure out about how to deal with it by then. But I don't think the community is big enough for this to be a widespread method. At that point, that amount of effort for earning a passive income would be commendable, really. And we could always turn the reward system off at that point if there was no easy answer. But I still think it would be worth a shot.
Lag problems.
If a bot that checks every half an hour or hour takes too much capacity or makes lag happen at random intervals, then either find a smoother coding solution or give up cause it's not worth it.