AHHHH, that's the key thing I didn't know (I have a Raspberry Pi sitting in a drawer and have played with it embarrassingly little -- I didn't realize how important having the SPI or other special interface is in this context). Thank you again.
With that said, though, there are also tons of inexpensive ways to output SPI or various other serial protocols from just about any device with a USB port, like your full-sized computer: https://www.adafruit.com/product/2264
The RPis and friends just optimize the workflow - theres nothing particularly magic about they way they implement SPI or GPIO, they just have it out of the box because its such a common way to extend hobby computer boards.
