The tour legs are "known" by FL4 as assigned flights, which means the web site says the origin, destination AND type of aircraft. I suppose (this being Alexander's area of expertise) that the type of aircraft sent by the web site is the one selected in the tour page.
However, as long as there is no automatic check between the "planned" and "actual" types of aircraft, this means that there is no blocker, but the flight is recorded as flown using the "planned" type of aircraft. There is of course a way to manually check for any abuse here, but as far as such a tour is concerned and the flight duration/speed/altitude are plausible, I suppose no one will ask further questions.
Thus, selecting an aircraft in the tour page is not mandatory for FL4, but I think it's good to do so in order to keep a clean flight record. Don't know for sure yet, I have completed the first world tour before the assignment system was created so I never checked this on my own flights.
Andrei