I have an Idea for a random source:
Lets take the previous blockhash and the Grain adress of the miner and hash it.
something like this: hash(previous_block_hash + Grain_adress).
The greedy miner would first have to search for a proper Grain adress for his miner in order to receive the nex superblock.
Maybe you can design it to be very difficult to find the next proper adress for the superblock, like:
if hash(previous_block_hash + Grain_adress) contains "99999" = superblock
You could also add a superblock difficulty:
if no superblocks are found for a long time, the superblockdifficulty goes down, lets say from must_contain = "99999" to must_contain to "9999".
You could make it very very hard for a block to be a superblock, but every single block could be a superblock and the greedy miner would be fucked.
lets call it: "Proof of luck".
You can't do this, as the computation needs to be exactly the same for all clients, that's where the network agree and form the blockchain. So you can't take Grain address into it.
Every client in the network can read the adress of the blockfinder from the transaction in the block or not?
You would verify a block by checking the adress of the blockfinder from the coinbase transaction and not by your own adress.
And you need to check the transactions. If not, then a miner could add any amount of coins to the coinbase transaction.
You can do that with the last transaction, related to the block creation. But there's no diff than you do it with last block hash. It's just a random number hash, you combine different values or not, no difference there. Anyone who want to write a program to check that can do exactly what you do in the client. No additional protection or encryption.