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Probably too much time on my hands – and it is cold in my North facing garage so my project can sit for a day or two. When you said you wanted to brace the swinging arm my immediate reaction was “Where would I brace it from and to?”
Most of the 80/90s braced SAs had underslung fabrications between the SA pivot point and the rear wheel axle, triangulating that relationship and stiffening them up so the above SA cantilever rear suspension mounts were structurally strengthened.
The M900 arrangement is different as the suspension is arranged so the input force is transmitted via rods and a rocker arm back (via an extremely strong pivot point on the frame) to the SA.
I think the need for bracing on an aluminium M600/750/900 should only be required:
1. between the SA pivot point and the point where the upward force from the wheel is applied to the SA (bottom shock mount), this would be a very tight little triangle and would not be easy to make as the difference between the points where the clamps grip the SA spindle and the centrally mounted shock are greater than the ability to reasonably triangulate given that you will probably have to be using 25mm x 25mm square aluminium tube and the triangle would have to be skewed centrally to the lower shock mount;
2. the point immediately below the hoop mount on the SA directly in line with the reaction required to operate the shock absorber and a line drawn to the centre of the rear axle in its mid adjustment position. On the left hand side this will almost certainly interfere with the chain run so will not be a straight line and therefore not be of optimal strength/ use.
If you made a brace between the rear axle and the bottom shock mount it would reduce any flex between the rear axle mount and the hoop mounting points on the SA (simply by adding extra section against the bending force applied at that point).
The brace could not be a straight line because it would have to accommodate the tyre width and also the chain movement on the left hand side.
Good Luck
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