Ruairi... one last thought. If you shoot HDRx you can use the "under" track for motion tracking. Then either combine that track with the "A" track and use a slider to determine just how much of it you want to use. Or... you can then ignore it altogether after you have used it.
Lots of creative options. Sorry if I sounded defensive... long day/week/month/year with not enough sleep.
Best.
Jim



