Would it be acceptable to use "at" instead of "cron"? That would be simple, especially if the job/script being submitted with "at" includes something like the following as the first thing to do:

at now + $interval this_same_script
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In other words, instead of using the crontab file to run a given script at regular intervals, use "at" to start the job the first time, and craft the script that "at" is running so that it schedules itself to run again after the desired length of time.

Running "at" at the beginning of the script being scheduled will make sure that the schedule doesn't slip too much with each successive run.