Ok - so it works, but I'm not sure how it works.
From the new IP range, I created a Proxy ARP entry for 1 of the new Virtual IPs (x.y.z.1/32), on the same physical interface.
Next, I created a NAT rule forwarding HTTP traffic from x.y.z.1/32 to an internal web-server.
Then, I tried connecting externally to http://x.y.z.1 - and I saw the web-page of my web browser.
So - great, it works! But what I'm confused about, is how it worked. Without having the new gateway specified somehow (since the Proxy ARP entry doesn't let you add a gateway), how am I able to hit this from off-site? Does this mean that my ISP has routed the IP to me?