From what I know about , and have since learnded about...there is nowhere to steam...Brute force may be my last option....Nurse,, the biggest hammer!!!
Once it's gotten to this point, I've never been able to re-steam, and make the dovetail wart move; this goes back to how tight the fit is. I suppose if you could see inside the joint, and get steam to that place, it might be possible.
There have been a few that have been so impossible, that I've elected to saw off the dovetail tenon, when the neck just won't come out. I glue it into the socket, install hanger bolts in the heel --CAREFULLY!--and continue the reset process from there. To saw off the tenon, I lay the guitar facedown, clamp so that the neckjoint opens, and saw through with a fine pullsaw. I've used a Fein Multimaster, a la Frank Ford's famous bonesaw, but not so much, anymore. That having been said, if I have to reset, say, a Takamine with an epoxied-in dovetail--yep, you read that right, and gee, thanks, Tak!--I'll use the Fein with a thin slitting blade, at least to start the cut.
Oh I almost forgot....but you may not like this....the thing that worked for me in getting that stubborn broken piece of heel out was to remove the heel cap and drill a hole into the lower cavity of the mortise and steam from there. I found that since the joint was so tight that the steam would get down there from the top.