I think you should approach this with a bit of impact, like a sliding weight you can put on the threaded rod then flip the frame upsidown and drop the weight down the threaded rod repeatedly (just don't go too hard on it and gradually increase the amount of impact towards a limit that you might feel is still safe for the stem and fork) and have it hit against a nut and washer on the end make sure you only hold on to the crown of the fork to isolate the frame from the impact. I think you can crack the corrosive bond between the steerer tube and the stem easier this way.
I'm afraid that your tool can start pushing too much pressure down against the top nut, headset bearings and down to the top lug and start buckling and cracking things down there. There still is danger of damaging the fork with the impact tool, but at worse it might just kill the fork instead of killing the frame which is harder to replace. But with some luck the impact method might get it out.
Just my suggestion/opinion........

Chombi