More importantly, you may have missed 7 month delay part as described below:
1 basecoin -> 1/b_(t+30000) USDcoins
1 USDcoin -> b_(t+30000) basecoins
Basically you can convert between the currencies, but with a ~7 month delay. The conversion rate depends on the backing level 7 months from now. The txn inputs and outputs remain unspendable until 7 months from now when the coversion rate is revealed.
There is an opportunity for arbitrage via conversion txns iff at time t
expected [market price of 1 basecoin at time t+30000 / backing of a USD coin at time t+30000 - market price of a USDcoin at time t+30000| information available at time t] !=0
(note that t+30000 is approx 7 months after you lock in your bet at time t)
In January, you can commit x basecoin to the purchase of USDcoins at a future price equal to 1/backing of a USD coin in August. You cannot back out of this. The spot market price of a USDcoin has 7 months to adjust after you make your bet. The backing ratio has 7 months to adjust after you make your bet. If adjustment occurs within a 7 month time frame (quite a reasonable expectation in my opinion), the bet has negative expected value (on average you just lose 7 months interest on the invested capital and plus any txn fee you paid to place the bet). If you have some information advantage that is the basis for your bet, the bet reveals this information to the public, giving the market has 7 month to front-run you.
Alternatively: You may think that you exploit the realization of the bet by manipulating the mining process. I agree with you. I would handle this by using proof of stake mining. Then if you want to manipulate price reporting, you have to purchase a major ownership stake in the system first. If you have an ownership stake, you no longer want to manipulate price reporting.
If you still see a problem, outline it explicitly so I can address it. I laid out everything in mathematical detail.
You suggested there was a problem (likely a figment of your imagination) and then asked me to guess what you might be imagining. This is not productive. If you are so confident, say something specific.