I've squirted liberally with releasing fluid, and have even tried a socket with 2 foot lever to try and break the joint, but it's stuck solid and if I try any harder am in danger of rounding the nut

One problem seems to be that, given the design of the top nut, squirting releasing fluid in the top doesn't actually get to the stuck threads, as it's more of a cap onto the stanchion with the thread inside the stanchion tube. I've tried squiring up from beneath the top mounting but I don't think it's getting there either. I also wondered if there was any way to push the whole fork up a bit through the top mounting just to get to the top nut thread joint, but given that I'm pushing up from the bottom against the suspension that didn't work.
Any bright ideas much appreciated, running around with 20 year old fork fluid of unknown level doesn't seem the best idea to just leave it.