I’ve a feeling the thermal expansion forces vs braking forces thing is wrong. Here’s my reasoning.
braking causes heating of the rotor braking surface which expands, so the outer ring grows in circumference. the spokes stay cool and hence don’t grow. the result is the spokes are in tension (a pure radial force actually), they are stretched slightly to ‘reach’ the expanded braking surface.
putting the rotor on with the spokes sweeping forward in the direction of rotation means gut feel is the spoke is in compression, but really it isn’t. As bencooper points out, they feel bending moment and shear force, a pure tangential force, totally perpendicular to the expansion force.