Currently (I don't know about the reverse HTTP Proxy in V8), your webserver must handle this after you DNAT all incoming web service traffic to it: 'Internet -> Web Surfing -> External (Address) : DNAT to Webserver' (leave 'Destination service' blank!).
I'd suggest the WAF feature as this is exactly what you are looking for. You'll have to use the v8 beta, obviously, which is not yet supported, so you'll do this on your own risk. You can configure your ASG to listen on a single IP and forward the requests in regards to which domain they address to different backend servers / services.
I personally for example have configured several aliases with my dyndns-account that point to various internal HTTP resources. By the way, port rewriting is also possible.
