After digging into the sources we were only able to find 2 ways of specifying the mailbox for a balancing pool:

• Using

akka.actor.deployment.<actor>.pool-dispatcher.mailbox-requirement = <mailbox trait name>

• Specifying mailbox settings in TypeSafe Config root

BalancingPool-<actor>

section
There should be some easy way to specify the mailbox configuration name to be used in the balancing pool dispatcher.