Hey Alexander, super interesting use case here.
You should be able to achieve it with LK Cloud. Ingresses are resources that can be started in re-usable mode. It'll only incur cost if you are ingesting a stream. So my recommendation is to create 100 ingresses, one for each channel.
After creation, you'll receive stable RTMP URLs. They will not change. To "turn on" an ingress, simply start publishing to that RTMP URL. When you disconnect the RTMP source, they will be "turned off" in standby mode again.