Hi João,
I never actually used AWS auto-scaling but in principle you should be able to use the Unattended Installation API to quickly boot up new servers.
For this I would have a standalone Controller outside the scaling group and add front-ends using the above API.
All you need is a proper image with platform pre-requisites installed, a place to put server.hsconf and private.key (s3 will do nicely), and a proper powershell installation script.
From my experience booting up a new front-end should take about 5 minutes (empty factory). A bigger factory may take some more.
If you have a proper health-check on your load balancer pointing to your application, you shouldn't have many issues with this.
Feel free to explore and share your progress / questions. I'll be happy to guide you through as needed.