Installing via WebPlatformInstaller cannot be done, where can i find good-old setup.exe?
What you can do is go to a machine that is connected to the internet but hasn't had the Web Platform Installer or Web Matrix installed.
Browse to the WebMatrix website and click the "Install WebMatrix" button:
On the next page click "Install Now".
The WebMatrix installer is dependent on the WebPI3.0 installer so that will install and you'll see a dialogue like this:
Click the "Items to be installed" link at the bottom left of the dialogue. This will open a window listing all of the components that need to be installed with links to their sources:
So you can grab these individually. You may need to install the full .NET Framework 4.0 redistributable as well if you haven't already got that.
One thing you will find is that if you want to create sites from pre-packaged Gallery Applications will need an internet connection. There isn't a way around that because it grabs the gallery product feed to display these applications every time you restart WebMatrix.