"Forking and execing prevents this."

And will be uselessly slow, which is why I dismissed it with hardly any thought.

     # time bash -c "for i in {1..1000}; do python -c '1'; done"

     real    0m9.351s
     user    0m6.439s
     sys     0m2.868s

No way will that be acceptable, and that's before we even load any of the cell code in.