I think the biggest issue with threading an unthreaded steerer is getting the die started straight. If you want to experiment a guy has a couple 1-1/8" threading dies on ebay super cheap right now, no pilots or handle though. It's heresy to talk about threading a threadless fork, but if it was mine I'd probably just try anyway.

Went and measured the longest 1-1/8" threaded steerer I personally have, about 8.5" and that's the longest non replacement fork I've ever seen. The frame it came off was gigantic by mountain bike standards, 23" or 24" at least.