The MSR fittings are a problem, I know. More recent designs from Primus and others feature a genuine rotating connection at the canister. MSR apparently gave up.
> The end of the fuel hose and the connector at the preheat end have plastic bits.
I'm not sure I understand exactly what you mean here. I assume you are talking about the Brunton stove stand?
Ah, well, that end gets hot under any conditions, doesn't it? But the 'plastic' you are talking about (inside the hose) is actually Teflon or PFA, and they will take very high temperatures. It's not a problem under realistic conditions.
The O-rings should be Viton. That can take high operating temperatures as well - over 200 C.
Yes, you could use brass washers to adjust spacing, but I think that places a high force on the screw threads, which would worry me a bit. I would need drawings of what you propose to comment further as I am not sure I understand.
Yes, the thermal feedback is sort of fixed. But the system can tolerate a very broad range of temperature, so precise control is not needed. As long as the preheat block is over (about) +20 C everything works fine. If it gets up to 50 C ... no worries. If it gets to 100 C I would back the fin off a bit, but even so that should not cause problems. You can fine tune the feedback by adjusting the length of the fin or the angle it goes into the flame too.
One thing to remember is that a small flame might not put quite as much heat into the fin as a big flame, but a small flame does not need as big a gas flow either. There is a sort of balance here between heat flow and heat requirement.
Why a 1 mm copper fin? I found that 1 mm x 6 mm copper fine worked fine. Aluminium has half the thermal conductivity of copper, so an aluminium fin 2 mm x 6 mm might work just as well. Do remember that aluminium melts around 550 C, so don't get the tip of the fin too hot! But yes, I have used an aluminium fin on other stove designs.
> brass vs. copper vs. aluminium bearings:
Sorry, more details needed. I'm not understanding just yet.
> a titanium pot with aluminium or copper fins welded on, but that'd be a trick.
Yeah. Actually, I suspect it would be a metalurgical breakthrough! Worth $$$.