This is going to take a little SQL-fu, but you can do it.
The secret(s) is/are the ${SQL: } variable.
The other secret is knowing that you always have the interfaceID no matter what kind of (interface) alert you build. So to grab the location, you would create something LIKE this:
(CAVEATE CODER: it's not exact, you will need to test this using MS SQL Management Studio to ensure it works as expected)
${SQL: select interfaces.location from interfaces where interfaces.interfaceid = ${InterfaceID} }