Thanks for this. The RNG is an area I'm still unhappy with, even having given it much thought so I very much welcome proposals for improving it.
My current proposal is that I will sign the blockhash that first appears on the network with a pre-determined key. The signed hash will be used to generate the random numbers is a pre-determined way. Without any choice over the key, nor the block, nor the signing method, I'll have no control over the signed hash and thus the numbers.
Of course, there are two possible attacks I might undertake - Firstly, I might prefer blocks that are favourable to my bets to sign, and so market participants should verify that blocks that appear first on the network are the ones that are signed. Secondly, I might generate blocks myself, but discard unfavourable blocks - this would require a large amount of the hashing power and would incur the cost of discarding blocks. I think it would leave an obvious betting pattern and would be uncovered by statistical methods.
I don't think it would be rational for me to undertake either of these attacks even if I were so inclined. However, I'd still like to remove either or both as a possibility so that less trust/verification is required.