You can connect the EVPL/ELAN to a LAN port on the MX and run iBGP between MX appliances across that private network.
Meraki MX supports iBGP only in VPN Concentrator mode (typically at the hub site), not in routed/NAT mode.
https://documentation.meraki.com/MX/Design_and_Configure/Configuration_Guides/Networks_and_Routing/B...
Meraki does not allow granular routing policies like Cisco IOS, it uses VPN route metrics and static routes with administrative distance.
If you want to prioritize the EVPL for site-to-site traffic, you can try creating AutoVPN tunnels over the EVPL (LAN interface) and the DIA (WAN1) and use VPN traffic control to prioritize the EVPL for internal subnets.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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