I have also encountered this problem, mostly with KYB forks. The fact that the inner tube bushing is "wedging" itself under the outer tube bushing is self-defeating: as it wedges under the outer bushing, it gets tighter and more resistant to breaking loose. Heating the area where the bushing is seated is a good idea, but on KYB forks there is an O-RING between the outer tube and the "seal casing" which is swedged on, so too much heat can melt this rubber O-RING and basically scrap the fork leg. Something that must be taken into consideration is the fact that the flat washer between the oil seal and slide bushing is always deformed (conically bent) once disassembly is achieved. What does this indicate? I would say that it indicates that the OIL SEAL tightness may be providing the resistance that prevents easy slide hammer disassembly. I mean, if the bushing was doing all of the resistance, much greater than the oil seal, wouldn't the washer be un-damaged? Wouldn't it be flat?
So with this in mind, it would make sense to remove the oil seal FIRST, then slide hammer the bushing out. HOW THE HELL CAN YOU DO THIS!?!?! Well, it can be messy, but it can be done: As soon as you detect this problem, (the inner and outer tube wedge solid when slide hammering it apart), stop banging on them. With the fork spring out, fill the fork with oil at about half stroke, re-install the fork cap with as little air space inside as possible. Now you have a SOLID fork (from half stroke, if you want you can fill it at full stroke, but it takes more oil). Remove the dust seal, remove the circlip. Now put the fork tube in a hydraulic press and compress it. The hydraulic power of the oil inside will force the fork seal up and out as long as it doesn't have a horrendous leak. Once it pops up, oil should not blow all over you and the shop unless you left a huge air pocket inside before you screwed on the cap. Once the oil seal is out, try slide hammering the bushings out again. They should come out a lot easier.