That's a lot harder than it seems, so, I have my doubts. It needs to not only run fast enough to bit-bang the signal but also simultaneously respond to bus requests from the 6502.
All depends on which microcontroller it is using. The Propeller microcontroller has multiple cores and specialized registers for video, so it can just dedicate independent core to video and bus control. An Arduino is certainly fast enough to bit bang video, and it could probably interleave bus control between servicing pixels, but the code and timing for that would get complex.
There's no graphical display on the apple I. You just need the atmel to handle the big banging. The rest is uart and there's quite a number of PAL and NTSC composite libraries out there that can do this for arduino.
The microcontroller doesn't go on the 6502 memory bus. If data is sent to the microcontroller over a serial channel (e.g, SPI or UART), it can handle incoming data during blanking periods.
