I think the old CB900c forks can be used and may need the triple tree as I have a Set of CB900 callipers on my present UK dual Disc CX500 ride and used the MC that I got with them as well(Since changes as it went bye bye).
and I don't know if there is one yours or US CB900s but there is a way round this.What you do is get a set of forks from the above with the discs and callipers of course.Then you run the right hand side line straight from the MC down to the right hand side calliper but use a Double Banjo bolt,
Remember also you would need the wheels as well. It's a lot of messing around for what may not be a great improvement for braking.
True, you've got to be in a physics thinking mode when you start trying to improve braking. I can only comment from some work I've done on my Pace car in the past that made a huge difference.
Pad brand, material used and even a single slot in the pads can make a difference. When braking a lot of heat is created precisely where the pad/rotor contact area occurs and the easier this superheated air can escape from that area the better the pad:rotor contact will be at any given clamping force. This is also why drilled &/or slotted rotors actually work. Also - if you think about it a bit - slotted is far superior in helping to vent the superheated gas, drilling just gives you some small pockets that will help to hold a bit thus only a small improvement.
On a single rotor system the best improvement is found if you can increase the diameter of the rotor but of course you have to have the proper caliper and pads to match the increased diameter. The reason this works is because by having the pads causing friction further out from the center of rotation you gain leverage.
Simply increasing the amount of contact area doesn't really do anything as your pad clamping power is just spread out over more area, the actual force applied per unit of area remains unchanged. This is similar to the reason putting wider tires on a car doesn't necessarily increase your traction and can actually decrease it, the "contact patch" area will remain the same - it will be wider but shorter in length as you've only got "X" amount of mass pushing down on the tire.
Nothing I'd love more than a larger diameter front rotor setup, especially if I could go dual rotors of a slightly larger diameter. Quite personally I think the single rotor brake on my CX500C sucks as far as stopping power goes but I haven't tried it out with the new EMGO pads yet. They have a single slot down the center of them so that will help to vent some of the superheated gas.