I played around with this a bit last year when I first got my HackRF. It's got quite the learning curve if you, like me, are not already knowledgeable about RF

I played around with this a bit last year when I first got my HackRF. It's got quite the learning curve if you, like me, are not already knowledgeable about RF and SDR. First step for your desired use case would be, I think, to acquire an SDR module that can receive broadcast FM and then it would simply be a matter of having your server command the SDR to tune to 99.9 MHz and then stream the result to the network.