I have clicked on "Give your feedback" and sebmitted:
"I need the ability to block a specific IP or subnet from using WAN2. This is because we have Cell Boosters that form a VPN to the cell provider and they whitelist the public IP. If the public IP changes (which it does when MX fails over to WAN2), they blacklist the box. Same could be achieved by allowing us to specify the WAN Interface or ALL on each of the L3 firewall rules (so I can deny the devices outbound traffic on WAN2)"