Dusting off a very old thread. I was just testing today on 8.12.0.2, and had the same issue. If you uncheck "Set username from dhcp option 12" in your AAA profile, then you can send a username from ClearPass -> the controller with MAC Auth at least.
Original Message:
Sent: Mar 03, 2014 01:54 PM
From: SethFiermonti
Subject: override username in Aruba controller from ClearPass
Going back and thinking a bit, there isn't a IETF "username" in a mac auth SSID so perhaps this cannot be overriden.