Hello joemajestee,
Here is something you can try by going under System > Dual WAN, select the WAN you want to edit and in the new window that populates you will see a field called Protocol Binding.
In this field click on 'add' and select all services, the Source start and end IP will be the IP of the PC you want and the destination will be 0.0.0.0 0.0.0.0 and the WAN will be the WAN you want the PC to use.
I haven't tested this but it may do what you want.
Hope this helps.
Regards,
Michael D.