That amounts to making a noise transmitter (pretty much what the old
spark transmitter was). No easy task for high power/ long range, & accurate harmonics.
You'll need one harmonic frequency for every 200kHz across the entire FM band (20MHz), making 100 frequencies in all, and making sure nothing gets into other bands.
And that's for the US channel allocations only, for european channel allocations you'll have to double that.
The Germans have a Traffic Announcement system already btw.. It interrupts even if you're playing tape/CD. A system adressing the individual FM stations is better.
But it could perhaps be achived the hard way with a carefully tuned duty cycle square wave, or a sawtooth wave.
Or perhaps it could be done by targeting the Intermediate Frequency used in most FM radios; 10.7Mhz, hoping the IF circuit is not too well shielded.
You'd have to do some serious lobbying to get such a brute-force transmitting system approved for use though.
A point is that it could actually be considered useless since many are not listening to a radio station to begin with.