Ok, you make it sound more elegant to just use the real RTC chip.
The Amiga isn’t supposed to set the RTC, so the write enable pin of the RTC could connect directly to the pic, which reads the GPS.
The pic doesn’t need to read data from the RTC chip, so the read enable pin could connect directly to the Amiga (except maybe for the pic to verify after write).
ie. RTC is normally connected to the Amiga bus, and can be read by the Amiga at any time,
except for a duration at startup, while the pic is waiting for a valid time & date from the GPS.
I was going to have the pic hold the rest of the Amiga in reset while waiting for the GPS anyway.
So I figure I’d just have to set the RTC once, and give the RTC it’s clock crystal, and allow it to take over for that power cycle.
I’m not certain of this, but it might be that while the Amiga is in reset, I can do what I want with the address & data bus,
and not need any tri-state buffer to disconnect the Amiga while the pic is using the busses.