Why do you need to write 8 bits in parallel, how fast is 'rapidly', and why must it be an Arduino?
Why Arduino? Because it's a platform I know and can write the code I need without going out and learning another system.
The 8 bits would feed an R2R ladder to generate an audio tone. How fast? I want to generate sine waves of up to about 2200Hz frequency. I've not tested it, but I suspect that grabbing 8 random pins and doing digital writes to them wouldn't be fast enough (nor simultaneous enough).
Why 8 bit? I'm guessing that that's the lowest resolution that'll work. Most people doing this task use 10 or 12 bit DACs. I want to see if an 8 bit ladder (with an output low pass filter) would produce a tone acceptable for the task.
The tone doesn't need to be perfectly pure, some, but not too much, distortion is ok, but I don't know how much. The tone would be used for AFSK modulation of a radio transmitter, and the digital signal would need to be decorate by the reciever. The only way I can think of to determine if this is good enough is to try it.