Take a look at this: https://github.com/ChristopheJacquet/PiFmRds
It's an software that generates FM MPX (with RDS) and uses Raspberry Pi's GPIO 4 (GPCLK0, Pin 7 (?) on the board) to output it to the sky at an arbitrary frequency. The output is square wave, so naturally there are harmonics everywhere. You can use analog low or bandpass filter to make it... "legal"
Now I was wondering if anybody that can program in C would take a look at the code, and help to modify it so that you could transmit straight from stdin without resampling, stereo generating or RDS modulating, making it possible to use external 192k audio interface of even Ethernet to push samples... from Stereo Tool's MPX output for example! Now it upsamples to 228kHz which is 4 times 57kHz, to match the internal RDS encoder I presume without heavy resampling. I have asked the developer for this a while ago but he just pointed out a way to do it. It seems that the code has changed quite a bit from that time so his instructions do not apply, I think.
Here's the issue that I posted in 2015: https://github.com/ChristopheJacquet/PiFmRds/issues/29
Could someone be so kind