Does anyone know if there is a configuration guide on how to configure the Draytek Routers for multi-homing on the WAN ports.

Primary reason is to emulate how the BT Business Hub works. The static ip addresses form one subnet with 1 for the router but in parallel to this the WAN port also picks up a standard DHCP IP which is used for the NAT mappings. This works really well so recognised hosts can be mapped to the static IP's and generic outbound traffic can go out over the random DHCP allocation IP and not clog / reduce chatter / attacks on the static ip's

the router so support it as they have multiple WAN interfaces and NAT but what you trying to accomplish?

I think you want to work with virtual wans and vlans.
Draytek seems to call LANs on different ports VLANs too, even though they may not be tagged.

Requirement is to provide 2 routes to the internet through 1 physical WAN interface.

1 route through a static network, ISP controlled but internally managed allocation of static range of IP's primarily for controlled inbound server communication. This is easily implemented either through physical or virtual WAN config.
1 route through a dynamic network, ISP allocated DHCP external IP for client outbound connections. This appears to be the problem.... how can the DHCP address be requested on a secondary virtual WAN interface?

Primary reason is the increasing attack vectors caused by users browsing to a website. i.e. when a new user goes to an external website, said website records the clients IP address and then attacks that address. These attacks increase by volume of user and a periodic update of the dynamic IP to a different IP relieves the 'attacks' temporarily.

I don't see why you couldn't make a DHCP request through 802.1q?
I assume the modem that assigns the address supports tagging.

There are NO vLAN's involved in this