I would be interested to know which exchange they will use to cash out.
None.
If it is anything like the billions of other forfeited assets they will announce ahead of time a day for an auction. They will auction off the coins and purchasers will wire money TO THE US TREASURY and only then will they send the sold coins. No counterparty risk, no waiting 180 days for MtGox to get their act together, no risk of an exchange getting "hacked" and the US government losing their $20M in BTC.
No possible way the US government using a quasi-illegal Bitcoin exchange.