If a group of machines do not run a proper cluster backend (e.g., Slurm) and users do not have root access, GNU Parallel is a good option.

It seems like an execution backend could be written to use GNU Parallel as a batch manager. It could use a temp file writable only by the user running snakemake as a job queue.

I don't see an alternative, unless there is a batch scheduler that can run as an unprivileged user (I'm not experienced with batch schedulers).