@Fry - I like your thinking as the forger can't do anything to change that (other than not forge which results in the not allowed to forge for a while penalty), however, it still means that the forger at least knows this value and so might decide whether or not to include their own tx depending upon whether they win (effectively improving their odds vs. non-forgers).
I see the problem with my proposal.
But having true random data would be great.
If we had true randomness on the Blockchain we could even do this:
For specific accounts we could set a random lock time for new outgoing transactions to be added to the Blockchain.
This would make Instant Payments for such accounts possible because an attacker would not know who the forger is, that can add the next transaction for this account to the Blockchain.