I'm surprised that Meraki has nothing in the documentation on how to determine this information as most ISPs now are locking their router/firewall/modems down to the first MAC they see. Maybe we need a feature request to see this info?
Just took a look at a couple of devices, an MX100, MX84 and MX64 and they all show that the MAC listed in the local webpage under the Security Appliance details is the LAN MAC. Same with the MAC listed in the dashboard under Security Appliance --> Appliance Status
Also, the WAN1 MAC is one higher than the LAN MAC and the WAN2 MAC is one higher yet. So:
LAN xx:xx:xx:bf:38:00 WAN 1 xx:xx:xx:bf:38:01 WAN 2 xx:xx:xx:bf:38:02