Well, I got curious to know if the theoretical is practical...
Yes, the method outlined above will work - but there is a big caveat. You have to have a complete full installer app.
What does that mean? Well, there are lot's of "Install OS X El Capitan.app" copies out there - in fact, there is one at the root of the system volume in Recovery mode - you can't use it to make an installation disk because it is a skeleton app missing some important code. In fact if you try you'll get an error telling you that it doesn't appear to be a valid installer.
The "Install OS X El Capitan.app" that ends up in /Applications when you download the .dmg from Apple and run the .pkg installer inside is a complete application and you can build installer disks form it.
So, you need to get the right copy of "Install OS X El Capitan.app" off the Internet. I have no idea how to tell if any given copy on the Internet Archive is the right one (I faked it by extracting a good version off an installer I already had and running the build in Recovery mode - downloading multiple copies of a 6.22 GB file to check them was way more effort than I was willing to expend).
But the gist of it is, get a good "Install OS X El Capitan.app" download from the Internet onto a flash drive and have a second flash drive you can make the installer from and a working Recovery partition you can boot into...and making an installer is entirely possible.