Interesting concept, wk.  I'm a bit confused how the "sell-a-share" thing would work in practice.  If I read your description correctly, if I wish to sell shares, I must have already mined those shares.  I'm not selling based on my hashing power, but rather what I've already hashed and recorded in your share log.

Is there a limit on when those shares can be sold?  For example, a share that is about to drop out of the 8N window is certainly worth less to a potential buyer than a share that has just been added.  I, as the seller, have potentially already been paid N times for an old share.

Using your hypothetical example of 1000 shares worth 1BTC, isn't that 1BTC over the lifetime of those shares?  Some of that 1BTC has already been paid to me as the miner prior to my sale.  How is a buyer supposed to know the age of the shares being sold, and whether or not the value proposition is there to engage in the transaction with the seller?