While I have not came across direct SDR implementation into a Raspberry Pi, I have been seeing more people taking the cheap USB Dongles and using them with Raspberry Pi for some small form SDR setups. Here is one example: http://www.rtl-sdr.com/tag/raspberry-pi/
I would like to see one too. RTL-dongles are just bad. With high noisefloor and only 8-bit sampling, dynamic range is just awful. I'm thinking of getting an Airspy, just because i don't have tools to debug and test one of my own build.
Airspy is only 12-bit, but i'd like to see 16-bit.
Have you looked at HackRF or HackRF Blue? I believe the Blue version has some cards that don't fully meet spec and are a tad less expensive. If a HAT could be made with HackRF capabilities that would tie in very nicely with Raspberry Pi.
