https://github.com/jlouis/safetyvalve/ provides exactly these features.
However sbroker master also provides a framework for building custom job regulators, there is even a property testing framework for the testing it. Its Erlang code but the simplest one limits the number of concurrent tasks: https://github.com/fishcakez/sbroker/blob/master/src/sregulator_open_valve.erl. The regulator process plugs together a valve, a queue and a meter, and has 3 different ways to request to run, blocking with queue, blocking without queueing and asynchronous with queue. The queues and meters come with sbroker but you would need to write the valve. I would really like a PR adding this feature and can help with the property based testing.
