I would imagine doing something like storing 12 decimal digits every 5 bytes (exploiting 10^12 <≈ 2^40) would be simple enough, great for random access, and goo

I would imagine doing something like storing 12 decimal digits every 5 bytes (exploiting 10^12 <≈ 2^40) would be simple enough, great for random access, and good enough at compressing (within 10% of theoretical optimal treating pi as a random sequence of digits) that it's not worth spending a lot of time trying to improve upon that. Of course, under a different set of constraints, one may find something better.