I've wondered if getting it extremely cold, like dry ice cold, or liquid Nitrogen cold if it would just fall off with a hit from a rubber mallet. I don't have any undercoating to remove right now or I'd try it with the dry ice.
Torch - Works
Heat Gun - Works
Hot/Cold methods - You will stress the metal and cause cracks. Not to mention that stuff is dangerous to handle.
Somebody else do it - That would be my favorite method! As I look at my current project, the worse shape ever, I am strongly considering it!
I also use Easy Off oven cleaner. Spray it on and wait a few hours then hit it with my 3000psi pressure washer. You may have to apply it twice but it will take it off. Also wear face protection. That stuff burns! On my last car it got it down to a point where it was easy to clean up with a wire wheel. You would want a rotisserie though. Wish I had one.
Here is an idea on a cheap rotiserie. My dad has had this done for hist last two cars, a 70 cuda and a 66 Charger. He goes to the exhaust shop and gets some pipe bent so it has two nice big bends and is wider than the car with the straight after the bends long enough to provide stability when the car is roller up on it's side. He gets some other pieces to put across it as bracing. He then welds some brackets to the device located so that the thing will bolt to the bumper mounting locations. Should work great for any unibody car. I might have some pictures of the one attached to the Charger somewhere. I'll see if I can dig it up later. Dad has a set up for a shain hoist in the ceiling of his garage and using that he is able to roll the cars on their side by himself.
I saw another "device" like yours but it seemed dangerous. It looked like it could roll back on you. Your system looks saver. I never thought about that. But I also don't have that kind of room in my garage to roll it over.
Dad does have a lot of room. I guess that's a side effect of building your own garage and not buying a preexisting one. My current garage isn't layed out in a manner that would work very well for rolling a car on it's side like that either.
If you don't own the equipment it can be very expensive. Quotes to blast the bottom of my Falcon ran from $800-$1000. But that is here in Atlanta, your area may be cheaper. When I look at it I really don't want to do this by hand. But I really don't have that kind of money either! But you are right blasting would be a good way depending on your situation.
A little bit - if you blast a lot of the metal with a pretty high volume blaster - enough that with nice shiny paint on it you will notice if it is a fender or flat panel. You can do small spots no problem, even on fenders and quarters, if you don't do it for a long time, and with relatively low pressure.
With a floorpan or wheelwell, you wont even notice. Buying a blaster and media is generally cheaper than having it done, but that rubber stuff is tough - it doesn't blast very well because it's bouncy, and the sand doesn't dig into it well. Though an industrial sized blaster won't care. It's not going to crumple up into a ball - and you kick your floorpans all the time.