Why not allow any amount between stated allowable min and max values, all to the same address? If the value is not acceptable, you treat it how you currently treat any non-0.25 value. I don't think you need to use different addresses or limit your users to certain round numbers. If you're willing to accept 'bets' of 5 BTC, why not also accept bets of 4.932345 BTC? That way people don't have to send multiple transactions.
You could even offer to split bets that exceed the maximum up into multiple bets of the maximum, and find some way to generate a different hash for each such bet. Perhaps by re-hashing the txid after appending the bet number to it. So instead of comparing the last digits of txid with block hash, you compare the last digits of sha256(txid + "0") for the first such bet, sha256(txid + "1") for the 2nd, etc. Then I can "invest" an arbitrary amount without spamming the blockchain.
Thanks doodlus
You really are smart. What do you think of the changes? I hope they will accept the 144 confirmations.