I saw @rosss got back to you on the Facebook group, but I will repeat here in case anyone has a similar question:
“Inside of Mocha, there is no “extrapolate” function. We used to have this in Monet/Mokey and is on list to add as new feature. Typically the way to track object offscreen is to stop on the frame where track goes bad, back up a few frames and then move your search shape to an area that is moving relative to the object as it goes offscreen. Track again from there. Another tip is sometimes switching the track option to just transform right before it goes off screen can help squeeze out a couple more key frames.”
I’d like to add to this and say you can also just use manual track to hand animate the track offscreen if necessary.