Hi Dan,
Yes, for sure. With the longer axle to crown (Surly fork 390mm, Thorn fork 378mm) the head tube angle will be slightly more relaxed. From your link, perhaps the effect of this should be to move the handling a little way along the scale further into neutral territory. I may try and work out the numbers over the weekend.
I've just ridden a few of passes in the Alps, and the handling was brilliant (Thorn 853 fork). I was up in the 60-70kph range quite often, and not the slightest handling problem/poor sensation. But when unloaded and, say, riding over some rough-ish ripple in the tarmac, if I look behind (for traffic) then I'd find the bike exhibits similar symptoms to those you describe in the high-trail section. When thinking about how I ride this bike, I'm now aware that changes of direction over less-than-smooth tarmac are something I have been avoiding; I ride over it then turn.
That there is a difference between loaded and unloaded indicates to me, in troubleshooting mode, that this (how the bike feels to me) isn't caused/fixed by a geometry change. I wanted to test my theory that stronger forks work better for me, as this was something that came up in my first impression of the bike. If, as a result, the geometry becomes more neutral, that's a bonus for me, as I prefer stability over feeling-fast through a manufactured 'lively' geometry. I get why people like 'lively' handling, but I actually go faster if I have more predictable handling (as I also do on motorbikes).
Your linked post is very interesting, I'll try and ingest it over the next few days. For the sake of completeness it would be good to test a stnadard Thorn fork, but as triasthete says, I've forked out enough already...
cheers,
Doug