It was more about making the test payload something that the public would be interested in. Normally blocks of concrete are used as a test payload. If you have to do the launch anyway for testing you might as well have some fun with it and having a convertible playing a David Bowie song while giving homage to The Hitchhiker's Guide to the Galaxy is certainly one way. And, intentional or not, it also gives a shout out to the opening of Heavy Metal.