How can I remove very stuck headset spacers (alu/carbon)

Based on Swifty's suggestion, I will make this a formal answer. In summary, I would (carefully!) use a high-speed rotary cutter to deeply incise the spacers and pry them open.

Note that it's probably simpler if you have a router attachment and a suitable bit for cutting metal; most of the rest of this still applies, but making the required cut is much simpler this way.

But if you're like me, and don't have a router attachment, I would start by making a makeshift depth guide by tacking a drill bit or some other reasonably solid piece of precisely-sized metal vertically to the side of the spacers. It would be sized so that with the shaft of the cutter sitting on it, the cutting disc doesn't quite cut fully through the spacer. This eliminates the possibility of a twitch of your hand damaging the fork.

I would then cut the full height of the top two spacers, extending down into the third but not trying (yet) to completely cut it. Since that is likely to abrade at least some of the cutting disc, I would reset and complete the cut.

(Actually, in reality, since at a certain point my time is worth a few dollars extra cost, I would start with a brand new cutting disc, and then instead of changing my guide, I would change to a new cutting disc - or two - to finish the cut.)

I would then try to pry the top spacer off, starting with a single flat-bladed screwdriver and twisting it, and moving on to a pair of them forced in opposite directions. (I'm sure you can MacGyver a makeshift tool with a pair of screwdrivers and a bolt and some wire.) If this fails, I would re-measure the guide and try cutting another fraction of a millimetre off.

The second spacer can be treated the same way, but possibly not the third. Even if you have a router attachment, you're unlikely to be able to safely cut all the way through the last spacer without damaging the top cover (seal). Of course if that is replaceable too, then you can just cut off the third spacer in the same way.

But if you need to/want to preserve the top cover, I would make multiple cuts in the spacer to create 2 notches spaced about 5mm apart such that I can grab that piece with a pair of pliers or grips and physically pull it away from the steerer. The idea being to break enough of the bonding to start to be able to move the spacer. (This might need to be repeated more than once around the circumference of the spacer.) Once enough of the spacer has been dislodged from the steerer, the remaining mess can be pulled off.