I got a GeForce 740 the other day ($80 refurb, 4GB DDR5) and started playing with CUDA - I converted the first bit of ld-decode and accelerated video FM decoding by ~4x - when the card is cool enough to run at full speed, that part gets to about 70% real time now. :)

With more work than I can probably afford to put in, and a Geforce 980 or somesuch that I can't afford yet, ld-decode could probably be made to work in real time.

(no updates on digital audio, but at least that's not too hard to capture from many players...)

EDIT: Installed the Nova S Plus I got. Looks like it'll work (with vmux=1 on the cxadc insmod line) but it has noise at 5mhz which will likely cause issues. *sigh*

On another front, it might be possible - albeit with modified firmware - to use some of the cheap USB FX2-based oscilloscopes as ADC's for this.