AFAIK, this is not possible in Simulink. The reason that dynamic selection of bus elements cannot be supported is because bus signals are an aggregate of signals that can each be of different datatype, complexity or dimensions - this means that the datatype/complexity of the output of the Bus Selector cannot be determined up-front at compile-time. Currently it is only possible to vary the dimensions of a signal, but not datatype/complexity, during model run-time. If all your bus elements have the same properties (and are either scalars or 1-D vectors), then you can use the "Bus to Vector" block to flatten out your bus, and then use the regular Selector block to select signals dynamically.
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