The OOM protection is done by procctl(2) in the background used by the protect(1) command. This tracing is not allowed in jails for security reasons.
If you're running out of memory you have bigger issues. But let's say you have a corner case where you are testing something and you do want to achieve what you've asked. From top of my head I'd say run the protect command on host as follows # protect -p $<pid of mysql daemon>.