I was pretty sure it was a PIC that their code ran on, but if it's got the requisite protocol laid out, I could start from there and cook up something suitable...
Hmmmm, I don't think I've heard about a PIC one if so, only an AVR. Are you thinking of the ADB chip? That's a PIC on the original boards, and the code for that is definitely floating about.
Did anybody have success with making one? I got the code from here and compiled it with make and avr-gcc. As I don't have a special avr programmer, I loaded Arduino ISP on a Arduino Micro and connected it to the Attiny. Using avrdude -v -P /dev/cu.usbmodem1101 -p attiny85 -c avrisp -b 19200 -U flash:w:/somepath/MacPlusRTC.axf I was able to successfully flash it to the Attiny. However, it doesn't appear to work now as I cannot measure the 1Hz pulse and I can also flash it again which shouldn't be the the case if the reset pin was successfully disabled.
Update: I was able to get a 1Hz clock using another Attiny and by appending -U lfuse:w:0xE2:m -U hfuse:w:0x5F:m -U efuse:w:0xFF:m to the avrdude command. However, my SE/30 still refuses to boot with this RTC chip. The curious thing is that it boots fine using a RTC chip from a Macintosh Classic...