My Thoughts on the subject:
There is no real 'safe' spot to put a jack as all the underside is designed to give a snag free surface, in the event of grounding on hazards like speed humps and ridge on the middle of a single track road etc.
I would recommend a block of wood on the jack to give a better grip on the floor pan.
When I was at the bare metal stage of my build, I welded a 12x12 box with ears at the rear to give a safe slip free location for my high lift jack, used when raising up 15" for placing on the build frame.
This also gives a safe spot for the trolley jack.
My recommendation would be to bolt a slip free jacking point, wood or metal, to the floor pan, as close to the centre and the rear as practical.
Lifting the rear of a Triking enough to get rear wheel out isn't going to be too dramatic, you are not going to be laying underneath, are you?