I have to wonder if this might be exactly the sort of application the Propeller MCU would be good for. After all, it has eight simultaneously-executing cores which can be separately dedicated to bit-banging their own sets of I/O lines. Dedicate one thread to handling the SPI traffic, another one or two to Mac-side communication, etc. Also... It'd probably require dipping into Assembly rather than Spin but the performance of the Prop might just be just high enough to ditch the CPLD. (I can't say that definitively, but I've seen some multi-mhz sampling speed claims out of the Propeller forums.)

But of course going there would mean scrapping the whole thing and starting over from scratch...