Hello,
There are at least 2 ways to do it:
1/ in JUNOS 13.3, MAC address per IRB unit is user-configurable, at least on MX. Never tried it on EX4300, but give this a try:
set interfaces irb unit X mac 02:02:02:00:00:00
2/ if the above fails, make one IRB unit a VRRP master with own IP:
set interfaces irb unit X family inet address 198.18.1.1/30 vrrp-group 10 virtual-address 198.18.1.1
set interfaces irb unit X family inet address 198.18.1.1/30 vrrp-group 10 priority 255
set interfaces irb unit X family inet address 198.18.1.1/30 vrrp-group 10 accept-data
For security-conscious, You can add VRRP authentication and block VRRP packets from going out by using output filter.
After that, address 198.18.1.1 shall answer ARP requests with VMAC. Don't forget to clear ARP on Cisco routers.
HTH
Thanks
Alex