Apparently there's some kind of hook in the original Mac's/Mac Plus ROMs one can use to get your code loaded at boot time. Of course, the code has to be sitting in an eeprom (or similar) at the proper address for this to work, but if you're plugging stuff into the ROM sockets anyway, this shouldn't be a problem. The problem is resurrecting that ancient bit of Mac hacker/programmer knowledge enough to know the specifics.
I ran into a guy at the Goodwill Computer Store years ago who programmed some of those old Mac products and we talked about it at length. From memory, there's some point where the Mac jumps the program counter to a high address in the ROM (I'm a little vague here) and then jumps right back or something like that. At that point, you can insert a chip at the proper address to intercept the execution. I think that's what he told me. I got his card. Perhaps I should email him and see if he wants to resurrect some old memories.