Using the blockchain is a good idea to prevent gpu mining in the long term (when it cannot fit in the gpu memory) because the slow transfer speed from host memory to gpu memory.
I'm not sure it could be a problem for realizing an ASIC. In the pipeline there is a step that picks up relative small amount of data from big set of data and fills the relative small pipeline memory (actually 130k is not small but in the future it could be). This is a bottleneck but not so big as for GPUs.
The problem with ASIC is not only the bigger calculation power than cpu, but the better energy efficiency. The problem with ASIC remain if it is possible to realize an ASIC not faster than a cpu but that requires thousands times lesser energy.

An idea: what about using the selector hash to vary the instructions sequence of second hash? In that case the ASIC pipeline becomes inefficient and similar to cpu.