Curious,
It has been a long time since I dabbled in electronics... but couldn't you also add a counter in between the arduino and the multiplexer? Then just use a single pin out from the arduino and step through the counter for the output you want?
Hmm, maybe you would need two pins from the arduino. One to step the counter and the other to tell the device to activate... that would probably mean an additional flipflop or something...
Cheers,
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