It's not possible with the default Veeam setup and proxy appliance, but there are methods to make this happen. Here are a couple of options:
1. If this is a one-time event, or at least something not very common, you can add a second NIC to the VM after it is powered on in the lab. Attach this NIC to the production network, give it a temporary IP, and there you go. Of course, if it's Windows, you need to take special care that it doesn't conflict with the production system names or you'll run into problems with two systems having the same name so make sure you understand exactly what you are doing.
2. Use a virtual routing appliance such as Vyatta or any of the good free firewall virtual appliances, connect one leg to the isolated network, and another to the production network, and configure it to allow access specifically to the resource you want. Then add a static route to the VM in the isolated network to access the production device via the virtual appliance. This method is quite safe but requires a little more setup, although done once, it becomes pretty easy.
There are actually a couple more ways based on exactly what you want to do, but option #2 is usually the best if it's something that you'll want to do more than once. Once it's configured the first time it's just a matter of manually adding a route to the VM in the isolated lab, which is pretty simple.