A client wants to send data (think 100 ... 10k baud) over a combined power / cable-TV coax cable, distances up to 1 km. The lower end of the spectrum (say up to a few MHz) is free, but the cable carries the 50 Hz 48 .. 60 Volt power. What would you recommend for this? I was thinking power-line modem chips, but those seem to be out of fashion (and hence difficult to get), and require quite a few external components. A DIY approach could be to couple a modulated carrier into the line and detect it with a PLL chip or even a PIC with DFT. But I hope chips exist for exactly this purpose?
Have you considered looking at the other end of the spectrum? Low-frequency communications, as you have rightly noticed, are not as common as they used to be.
There are many chips around that handle all the modulation, demodulation, etc at >1GHz - take the MRF24J40MC from Microchip (maybe not the cheapest solution) for example - 2.405Ghz to 2.475GHz. Or the ADF7241 from Analog Devices - again in the 2.4GHz range. There are many many more.
Yes, I know these are aimed at wireless communications, but then, what is Cable TV if not wireless communication with no aerial? The cable is basically a closed loop aerial between two points. A modulated high frequency signal being sent between two points through a medium - just using copper instead of air.
Since you mentioned ASK in a comment, the NXP TDA5051 is an ASK modem designed for home power networks, and operates at either 600 or 1200 baud, which is in between your 100 .. 10K baud range. It is RoHS compliant, and is available at Digi-Key for under $3 in quantity. The interface circuitry required seems to be fairly straightforward.
Have you considered just connecting some v.90 modems? The 50 to 60 Hz is below the audio range those modems use, so can be filtered. The television VHF and UHF channels likewise are outside the modem frequency range. The modems are 600 ohm impedance and signal in the 300 Hz to 4000 Hz range.
Telephone wiring (600 ohm) to CATV wiring (75 ohm) will require a suitable coupling transformer as well as a power filter that blocks the power frequency.
Telephone ring detect and onhook/offhook features won't work, of course, you'll need to give the modems commands to force connection.
You've got a megahertz of bandwidth available and you only need 10 kbits/s of data? You could just use a series capacitor or RLC circuit (basically, a bandpass filter) to couple transmitted data onto the line and a 200 Hz high-pass filter/1 MHz low-pass filter combo to filter the received signal, then do a software modem with a 100 ksample/s or so ADC and DAC. Higher-end (96 or 192 ksample/s) audio codecs might do the job nicely. You could do it with a dedicated low-power 32-bit chip or similar, which might even have good enough ADC and DAC on board. If it has PWM outputs instead of a DAC, then just use them to drive a push-pull stage to get a class-D transmitter.
Assuming you have the people needed to write a modem, of course. But, again, the excess of bandwidth means it doesn't have to be a particularly good one, just one that can deal with the cable irregularities. ^_-