"..Question for you: is the above still correct if you hold accounts at multiple brokers? Would a separate ACB not be calculated on a "per account" basis? If David had bought the higher priced CUU shares at a Broker X could he not sell only those shares and claim a loss based on that account's ACB?.."

The only accounts that you can calculate a separate "average cost" are the special accounts such as the TFSA and RRSP accounts. Shares from all other accounts for the same type of shares have to be pooled to calculate the "average cost". JMO GLTA!