You have to completely explain your idea about random digits- I have no idea what you mean - please explain. You have to realize, when I read that, I take it as : A) The user is typing something when they mine, or B) You are trying to hash suffixes in a block. I think we need the verbose description of the idea.
Sorry I am not explaining myself. You take the hash of the miner's reward address. The last number/letter of the hash has to match the block that will be generated. That gives you 1 in 16 chances that you mine the block.
Hmm.... I think this scheme would only work if a miner cannot change the reward address mid-block. How do you guarantee a miner is not creating more receive addresses while mining a block - by pre-testing the solution with the first createblock to see if its going to "win"? What mechanism could be used to pre-determine a registered miners entry point into that block? Even if we made each miner pre-register to mine the next block, what would stop a miner from pre-creating 16 receive addresses and starting the miner with all, and as soon as they discover which address will be rewarded, they increase the hashpower on that address midblock?
EDIT: This is why we keep coming back to UTXOs (and CPIDs) - these are the only things that so far seem to hold up. (CPIDs less so, but generally, people want to keep their RAC and its work to add a new CPID and generate RAC - of course under certain circumstances).