On Chenghou reverse, let your ship drop into the abyss on the boost shortcut to the right after the magstrip. 1 out of 5 times or so, you will respawn upside-down at the start of the magstrip
On Moa therma reverse, get someone to push you out of the track (or do it with a boost) on the turn before the last magstrip section. Often you will respawn upside-down there as well