I use a harbor freight jack, as does my son, but not the one illustrated. His buddy had one but it did not hold up. We use the all steel version. Heavier but holds up better in our experience. We used my sons to jack his deck into position recently. We were lifting most of the weight of a 16x16 deck with it.
I would not crawl under a car with a HF jack stand on the edges of the plastic pockets. You are going to damage the pockets at the minimum and they could break IMHO. If you don't have the tools to make plywood blocks and don't know anybody that does, and don't want to get a vibrating saw, how about buying a foot of 1x1 inch steel rectangular tubing. If you don't know anybody locally that sells it you can get it from McMaster Carr. Cut it with a hacksaw to a length that will fit into the lifting pocket and lift away.
I do not put the metal cup of my HF jack directly on the front or rear lifting point. I have a disk of plywood in there. That is a cosmetic thing, however. I don't think direct metal contact with the bimmer will hurt it but it could smear the plastic or paint a little.
128i Convertible, MT, Alpine White, Black Top, Taupe Leatherette, Walnut, Sport
Ordered 5/22/09, Completed 6/4/09, At Port 6/9/09, On the Georgia Highway 6/13/09, Ship Arrived Charleston 6/24/09 at 10pm, PCD 7/21/09