That's probably going to be easier than getting the old one modified. Not to hijack the thread, but I'd be happy for any leads to frame builders that would be be willing to take on a job like this. I don't need a super-elegant custom job--just a good-quality lugged fork (the tubing is plain old 4130 steel, I think) that duplicates the geometry of the original.

This might be nuts, but as a guy who works closely with machine shops, it seems to me that it might also be possible to make some sort of double-ended expandable internal plug fitting that would allow one to cut off a threaded steerer a couple of inches above the fork, and mechanically fasten on a threaded steerer from there upward.

That would not be a trivial design project, and it would have to be executed with precision, given that the cost of screwing it up could be very high. A custom fork is probably a better idea all around.