C4D displacement actually combines using Alpha channel, but it works in a strange way in that 0-99% alpha is different than 100% alpha.
But disregarding that, if we concentrate only on the 100% alpha behavior, the topmost (rightmost) material overrides the displacement even if the displacement channel is turned off! This makes creating decals, for example, impossible.
So what we would do (and it would mostly be compatible with C4D's solution) would be to linearly blend the displacements of all materials with displacement enabled on the stack. If all materials had 100% alpha (or disabled alpha channel) and enabled displacement channel, this behavior would be the same as in C4D. But at the same time, it would be more logical and flexible.
What do you think about this idea?