2nd IP for outbound / established connections only

My website has been the target of a DoS attack and as part of damage mitigation I've disabled services that are vulnerable to identification attacks. This includes my forum's image proxy.

In a typical situation:

1. User adds an image from nohotlinkz.com

2. Image is broken for everyone and they have no idea.

Using the image proxy:

1. User adds an image from nohotlinkz.com

2. Image is downloaded by the proxy to the cache.

3. Viewers access mywebiste.com/proxy.php?hash=imgcacheid

That way, the viewer is guaranteed to see the image. It also helps when you hotlink an image from a website like 4chan, which has a very short life expectancy for images.

However, because of the DoS attacks, I've disabled this. If I upload an image from http://iplogger.org/, the server request is stored and it shows the webserver's true IP.

I'd like to add a 2nd IP for my website. This IP is only used for outbound connections and all inbound connections not already established get dropped. I'd also like to make it specifically for port 80 and 443. That way, it's as secure as possible.

1) Is it possible to do this at all? I assume it is.

2) Is it possible to do this with another Linode's IP? I already have a server dedicated entirely to vulnerable processes. If I could just borrow its IP (and perhaps share it between 2+ servers), that would be the most ideal solution.