I do not really believe in this story. The operations described here relate to operations with a high degree of risk, which means banks would be forced to form a reserve for them, and therefore double the financial burden. Its another thing if we are talking about the owners of banking groups that operate with their personal assets, then it is quite possible.
risk is banks putting their funds into other companies which then play against customers funds.
the banks DO do OTC. they only use exchanges for smaller amounts.
the volumes of exchanges cant handle real OTC trade values