When will the proof be posted of the unsold BRX being destroyed?
The proof is in "getinfo" in the wallet. If you open the console in the wallet (Help>Debug Window, Console tab) and type 'getinfo' you will see:

{
"version" : "v1.4.2.0",
"protocolversion" : 61008,
"walletversion" : 60000,
"balance" : ******,
"balance currency" : "BRK",
"staking balance" : *******,
"staking currency" : "BRX",
"newmint" : 0.00000000,
"stake" : 0.00000000,
"staking moneysupply" : 6268082.49402536,
"staking totalmint" : 6268082.49402536,
"blocks" : 7938,
"timeoffset" : -17,
"moneysupply" : 17239798.20328000,
"moneysupply currency" : "BRK",
"totalmint" : 17239798.20328000,
"totalmint currency" : "BRK",
"connections" : 17,
"proxy" : "",
"ip" : "lchyxhgaxjpar2bo.onion",
"difficulty" : {
"proof-of-work" : 1107.78512449,
"proof-of-stake" : 457549.58290176
},
Where "staking moneysupply" is the total available to stake.
12,500,000 - 6268082.49402536 = 6231917.50597464 burned.