I know you can call Windows Services from a Windows store app as described here, so I was wondering if it would be possible to install a custom one packaged with a Windows store app.
Thanks in advance.
Did you mean a WCF (web) service vs. a Windows Service (the latter isn't addressed in the link you referenced). Regardless, your application will not pass certification if it requires a loopback connection to a service hosted on the same machine.
You can set this up for testing, but note:
Enable loopback for network access
Network communications using an IP loopback address cannot be used for interprocess communication (between two different apps) since this is restricted by network isolation. Network communication using an IP loopback address is allowed within an app within the same process for communication purposes.
A developer may want to use loopback for testing or debugging purposes, even though that capability will not be available for customers. For example, an app may want to download data from a web service. For development purposes, the developer wants to test the app on a single computer that is configured with the web service locally on 127.0.0.1.
Warning Loopback is only permitted for development purposes. Usage by apps installed outside of Visual Studio is not permitted.