How can I access a VirtualBox host-only network from outside the host machine?
Turns out I forgot a step... I needed to add a route to the virtual guest telling it how to get to the external LAN my appliance is sitting on via the IP address of the vboxnet0 interface on the VirtualBox host. Once I did this, I was able to communicate with it externally.
thanks @Sirex for commenting on my question enough to make me think about it more!