Ok, figured this out. You CAN redefine preset softkey. The challenge is you must find the menu number of the preset softkey you wish to define because they don't seem to be documented in any of the readily available docs. In this case the AUX CTL menu is #52 (which I'll explain how I found later). to redefine a specific softkey in that menu the formula is menu# -1 * 6 + softkey# . In this case it's 52 -1 * 6 +2 = 308 and then use the KEYDEF command with that key number.
Cheers,
Ron