This is my next course of action. Good excuse to add another set of randomly useful tools to the toolbox.

I don't understand how this would work if the bolt is actually threaded into the caliper carrier, which is BETWEEN the bearing carrier (no threads) and the rotor. I'd still have a bolt with no head stuck in the hole, holding the caliper carrier in place...

The carrier bolt is #3 in the link below:
RealOEM.com - Online BMW Parts Catalog

Once you dremel the bolt's head off, the carrier can be removed.
Now you end up with a stud. With heat, Vise-grips etc., it will come off.

I actually thought about this after my last post...I guess the caliper carrier would just come off WITH the rotor. Yeah, duh on me. It's still Monday, right? I may still try the "biting" sockets first. A set only costs $20 and would be a good thing to have in the toolbox. If that doesn't work, I'm going at it with my 4" angle grinder. (I have a Dremel, but don't think I have a single bit that would be up the the task.)

The trick which did my job was the floor jack. Jack the ratchet up on the end, and God knows how much force was on that poor ratchet, but BOOM! It worked!

My only question is if there is enough room to slide the carrier bracket off the now stud before the bracket hits the face of the rotor?

Never had to do this so I don't know.

This link should help. MOTOR Magazine Article | MOTOR Information Systems

I would exhaust all other efforts before cutting the head off the bolt.

My experience with drilling out a bolt has not been good largely depending on the hardness and diameter of the bolt. It has been at least a 50/50 chance the first few drill bits will break off making the problem much harder to fix. I have had success with heating the surrounding area (with flame wrench) and cooling the bolt if the bolt was fairly large and when I had an excellent fit on the bolt head coupled with a breaker bar and a big hammer hitting it in both directions. If the bolt head is well rounded I have ground the flats and driven a smaller 6 point socket or box wrench. The very best solution for me has been to keep soaking it with penetrating oil several times a day and let it sit overnight and then use the breaker bar and big hammer method.

Try soaking it with a mixture of 50%ATF and 50% Acetone.

This is generic info re Tight Bolt...

1. Rust or Loctite creates tight friction on the threads themselves. In this case, cutting the bolt's head off makes no difference, simply because the threads are bonded together.

2. Bolt tightened too much at factory.
In this case, cutting the bolt's head off instantly releases the clamping force, which is basically between the head and the surface that the heads touches.
The stud is now free to move.

I have had this issue (too tight caliber bolt) in many cars. Once the bolt is removed, it is obvious that there is no rust buildup.
So the cause for tight bolt was not rust but rather the impact wrench used at factory that might have been over torqued.