I've been poo-pooing using Raspberry Pis as WFM transmitters, but VK5DGR has recently been experimenting with using them for FSK, and hooking one up to a spectrum analyzer. Turns out they're not as bad as one would think! You always need a low-pass filter for the harmonics of course, but there's seemingly zero splatter.
http://www.rowetel.com/?p=6317