Hi Simon,
Thank you for your feedback. I have reported your need as a suggestion for future development of our app. It seems like the feature is among the top requests, but it is not in the pipeline at the moment. We all agree this feature would be a great addition to capture. You can also follow this post: https://community.pix4d.com/t/3865-Pix4D-Capture-Follow-the-Terrain-option-REQUEST