Thanks guys, theres a lot of food for thought here.
@Murray_Calaverato your points re. safety, I do plan on putting my seatbelt over the top of the harness (as is required by Wakefield) so the baby seat mounts might still be an option as I don't intend for them to hold my 115kg lump in a crash. Not sure if that changes your mind on them?
I was also worried about the spine compression when your shoulders go above the seat (which mine do). This issue makes me think that I should get a seat also before using the harness with stock seats.
@mlr I couldn't get the videos to load on that site, but I found the one below. Very interesting. It reminds me of the seatbelt locks that you put on the top of the belt to stop it moving.