I have been really scouring /proc and /sys to find an answer for you, I have found the location(s) in /sys/ where it is stored, but the address changes in those items when you change it via ifconfig.

The location of the file is /sys/class/net/{interface}/address

Since it appears that it cannot be retrieved once changed (without a reboot) you can write a script to store the contents of the file listed above and then make a reference to it when you want to change the value back.

#Store the existing ethernet address

cat /sys/class/net/eth0/address>>~/HW_add.txt

#restore the MAC address from the file

ifconfig eth0 hw ether $(cat ~/HW_add.txt)