BP- yeah, I looked at steel, and could easily make it work for just a bit more weight (and a bit less money, and maybe easier fabrication). However, I think aluminum is a good choice IF you are comfortable working with it and paying for it (which I am - I'm not planning anything fancy, and I'm looking at well under $100 in tubing, which isn't out of line with other costs; carbon fiber was WAY to spendy for the dims I needed). My impression is you can get a stiffer, stronger structure from the same mass of aluminum IF you are able to use fatter, larger tubes (or deeper girders, etc). If I end up needing tubes over 3" od, I'll probably go with steel, but I'd like to keep the fork weight at a minimum to reduce unsprung mas.
For equal lengths and loads, it looks (from those same online calculators) like 2x1x.125 rectangular steel is pushed closer to its limits than a 3(od)x.25 aluminum, and they weigh about the same, so it sounds like I'm on track. I'm not sure what bits would NOT be subject to fluctuating loads - I assume you mean the frame itself, maybe the bushings & pivots of the control arms? For me those may go either way, since their weight doesn't influence the suspension behavior as much. Steel looks appealing for the control arms just because it is easier to weld, and costs less (so I can afford to build multiple generations to improve the design). The idea is to have a nice, light, strong fork that is adjustable enough (ball joints go into "triple clamp" type setup) to allow me to set the geometry after the fact.
Brass Machine - The duo lever design is hard to replicate (compact construction w/ monocoque and cast parts) and doesn't lend itself well to adjustable geometry, but otherwise is a Hossack setup (according to both BMW and Hossack). Cosstinos setup looks quite similar to what I have in mind, and is much like other prototypes (including Hossack's own racing bikes and BMW adaptation) I've seen photos of. His geometry looks a LOT like what I had in mind- steep steering angle, a few inches of trail.
The brake mount in his later designs is interesting- it goes at the bottom of the fork, under the axle. Not sure why he did that- I'm guessing it moves the brake as close as possible to the center of the steering axis (which is actually why its in back on normal forks). Looks trick, might be worth a shot.
The main differences between all those designs is the length / mounting locations of the control arms, and the mounting location / actuation method for the shock. Cosstion looks to have some sort of rising rate linkage going, which I had considered. I've got the full rear shock and linkage from a GSXR to play with, so the details would be a bit different, but much else looks similar (same rough control arm length, rod end pivots for control arms, ball joints sandwiching fork bridge). My setup is just gonna have a hell of a lot beefier fork, since I'm working on a bigger bike. That, and it will be bolting to an existing frame with an existing headset, so the steering will work differently.