Like the Test-installer form I need a Command-Line textfield for building. Then I can pass parameters to the building process, without changing them in the project. Then it works like the using InstallJammer from commandline to build new installer.
Then I use the Virtual Text's value in the project as default value. If I don't pass any commandline arguments, it will use the stored value from project. When I change the Virtual Text and I forget to set back than the wrong value will be the default setting.