IOW, while I run crontab -e on bookworm, inside my emacs session, I want a subshell to run crontab -l, but the latter has to run on bullseye in order to pick up the old crontab. I'm not sure how I would do that.
Try running : ssh user@bullseye crontab -l It will locally list the crontab from remote user "user". Note I've never used emacs, so dunno if ssh is allowed !