Well… I did get it working, so for anyone wanting to try this out, here's how:
1/ make sure you get an OpenVPN client connection working
2/ assign an interface to this connection via Interfaces->Assign->click the + symbol->assign the interface to your OpenVPN client connection (typically ovpnc1)
3/ configure the interface: click Interfaces-><your_new_interface>->enable it & give it a sensible name (let's call it "MYVPN" for this example), choose "None" for IPv4 & IPv6, leave all other fields blank
4/ assign another interface to one of your physical ethernet ports (call this one "SECURELAN" for this example, and let's assume the physical interface is called re1)
5/ configure it again by enabling it and giving it a static IP (192.168.100.1 for this example)
6/ enable DHCP for this last new interface so your clients can get an IP address: Services->DHCP Server->SECURELAN->enable the interface & specify a DHCP-range (range 192.168.100.2 to 192.168.100.5 for this example)
7/ click Firewall->NAT->Outbound tab and add a new rule: select "MYVPN" for the interface, source = network 192.168.100.0/24, destination = any, translation = interface address
8/ choose "Manual Outbound NAT rule generation" (IMPORTANT!) & hit save & apply changes
9/ click Firewall->Rules, pick the "SECURELAN" tab and hit the + symbol to create a new rule: interface = SECURELAN, protocol = any, source = any, destination = any, gateway = choose MYVPN
And you're done 8)
Test that everything is working fine by connecting a client to your re1 interface with a LAN cable and doing a traceroute to a url of your choice.
I'm going to be finetuning this a little more to check for DNS leaks & such, will post again when I've verified this.
Hope this may help anyone wanting to route some traffic over their OpenVPN client connections. \m/</your_new_interface>