Well, I thought about doing it in software (bit-banging the GPIOs should probably be enough for any kind of signal at 300Hz)... I really need to read the spec,

Well, I thought about doing it in software (bit-banging the GPIOs should probably be enough for any kind of signal at 300Hz)... I really need to read the spec, but I guess this signal is just needed for the handshake? If that's the case, you can probably afford to do it on the CPU. As for the open source part, yeah, I was specifically thinking about open ASIC designs (whether at the RTL level, thus compilable to an FPGA, or at the layout level). The ecosystem is blooming, with open source memory compilers, as well as academic/enthusiast tools (BAG, scala, hammer-vlsi, migen) that are making great strides.