Payload
OK, I may be wrong and/or the question may have already been asked and answered.
Most people who do long-distance stuff in an SEP minimise payload so they can maximise fuel (see MK and his 5 Gal ferry tank on the front seat).
So, when planning Africa and Australia there must have been a decision point whereby the whole route was planned around standard fuel (i.e. the stuff that is is the normal tanks) rather than the advantage brought about by having a ferry tank installed. This is actually core to the whole trip and would have been a very early planning decision. As Artemis is an N reg aircraft, there aren't too many regulatory hoops to jump through in order that you could add 85kg of fuel where Ewald ended-up sitting. 85kg = 120l = quite a bit more airborne time = longer legs.
My question - why didn't they go for range when scoping/planning? Was this due to the aspiration to have numerous "outreach" stops or was it because they ordinarily expected to have 85kg of pink flesh (dead weight) sat in the front?