There is normally no issue with the MAC address being the same, because the WAN VLAN and the other VLANs are separated Layer 2. My ISP requires cloning the MAC address of their modem router, so I set that MAC address on the bridge.
The only downside is when you have two or more WAN lines, and the ISPs all require specific MAC address. In that case you'll need to add a MACVLAN interface on the VLAN interface (it's possible since 7.20:
before 7.20 it was only possible to put that on the parent bridge or an ethernet port) and use the MACVLAN interface as the WAN interface with arbitrary own MAC address.