Your hoops are the same as mine then? I was going to weld some flat bar onto the subframe, and then use a 6-point bolt-on subframe on each side for the cases, with a brace behind the number plate (although I will take the plate off and rivet it onto my topbox I think ... less likely to break w/ rocks). This will all be made from mild steel thick-wall tubing. Although the joints will all be welded, the frames will bolt on, and therefore be removable. I think a major fitting point will be the passenger footpeg hangers actually. Perhaps I will have a play in photoshop and post up a rough geometric plan (like I did with the frame strengthening article...)
I also considered soft bags, but with some of my journey in Eastern Europe and quite a few valuables, I couldn't take the risk. I can also use hard cases at home! I think Dan Walsh (from DiRE magazine) used soft cases and found them OK, although burning is a major issue I think. CS recommends a small lockable tool case on the rear carries as well - your ammo cases sound perfect for that.
I think I am a bit out of the ordinary, in that I want to avoid using a tankbag. I have never liked them, and will instead use a duffel on the seat behind me, with a map case on the 'bars. This should be fine with my GPS and stuff.
I can also rig up a bag over the screen, fender-bag style. A headlight guard / front rack would be great, but shipping from Riky Cross is outrageous. Will get my G/f to get a quote (she lives and works near Turin). Perhaps we can club together to buy some stuff?? Re: making your soft bags slash-proof, you can buy (from YHA and other places like that) special mesh with lockable closures, designed to fit over backpackers bags for just this purpose. Look into that, and save yourself some work.
I think I have bought some 2nd hand crash bars for a 3AJ from australia (!) and so you're more than welcome to copy the design if / when they ever arrive. A guy I know had them, and they ceased fitting his bike when he added a kick start. I am going to graft on a small jerry can rack onto each side, to carry water and fuel. Will let you know, but engine and TANK protection is vital, esp. on the twinlamps.
Good to hear you called X-bikes. My friend has had his engine back - great work but very expensive in the end. I don't think the parts were cheap... Unless you stress that it's urgent, I think Chris also tends to <ahem> take his time!
Let me know, whatever. Perhaps we should meet at some point? I am based very near to Greenwich, and ride into London (on the XT of course!) every day.