Even though the ladder may never slip, I would be worried about the weight capacity of the ladder, and how well your tie downs or ratchet strap can hold weight over time. I would at least tie the straps off to make sure they aren't going to slip. The stands are obviously a better way of doing things. Even a cheap stand would be better than doing the ladder thing.
All that said, I would be pulling off body panels before I did anything like that. One slip of a strap or someone bumping into the ladder, and both of those lowers are screwed along with whatever else hits the floor if it comes off the rear stand (possibly everything)