Is this going to be feasible? Would having an expander (or rather, two expanders chained together) be a better plan, even though each channel would only have 1 pixel? Is there any utility in putting a resistor after each pixel? What is the recommended maximum length of data line between two of these sorts of pixels?
My experience suggests there’s no hard and fast rule about the length that will work vs. one that will not. It can depend on the wire gauge and brightness used (and thus voltage drop) on the power wires (affects the quality of the data re-transmission at each pixel), whether the data is carried on a twisted pair, the baud selected, and whether that particular length is going to be a harmonic resonant frequency for it. This is why testing is the best answer.
I certainly know some people have a rule of “no more than X ft to first pixel” and I’ve heard X in 6 to 20 ft. So you’re right in the ambiguity zone. Search the LEDs Are Awesome group on FB to consume the diaspora of opinions.
I think the output expander definitely decreases your risk by a lot. It’s how I would do it for sure. I think you would only need a single expander, right? 8 channels, 8 pixels?
At 3W (600 mA) per pixel, you definitely need a separate power bus - sounds like you plan for this already; don’t run power through the expander.